## Saturday, October 24, 2020

### Silicon nanoelectronics is a truly extraordinary achievement.

Arguably the greatest technical and manufacturing achievement in all of history is around us all the time, supporting directly or indirectly a huge fraction of modern life, and the overwhelming majority people don't give it a second's thought.

I'm talking about silicon nanoelectronics (since about 2003, "microelectronics" is no longer an accurate description).  As I was updating notes for a class I'm teaching, the numbers really hit me.  A high end microprocessor these days (say the AMD "Epyc" Rome) contains 40 billion transistors in a chip about 3 cm on a side.  These essentially all work properly, for many years at a time.  (Chips rarely die - power supplies and bad electrolytic capacitors are much more common causes of failure of motherboards.)  No other manufacturing process of components for any product comes close to the throughput and reliability of transistors.

The transistors on those chips are the culmination of many person-years of research.  They're FinFETs, made using what is labeled the 7 nm process.  Remember, transistors are switches, with the current flow between the source and drain electrodes passing through a channel the conductance of which is modulated by the voltage applied to a gate electrode.  The active channel length of those transistors, the distance between the source and drain, is around 16 nm, or about 50 atoms (!).  The positioning accuracy required for the lithography steps (when ultraviolet light and photochemistry are used to pattern the features) is down to about 3 nm.  These distances are controlled accurately across a single-crystal piece of silicon the size of a dinner plate.  That silicon is pure at the level of one atom out of every 10 trillion (!!).

This is not an accident.  It's not good fortune.  Science (figuring out the rules of the universe) and engineering (applying those rules to accomplish a task or address a need) have given us this (see here and here).  It's the result of an incredible combination of hard-earned scientific understanding, materials and chemistry acumen, engineering optimization, and the boot-strapping nature of modern technology (that is, we can do this kind of manufacturing because we have advanced computational tools for design, control, and analysis, and we have those tools because of our ability to do this kind of manufacturing.)

This technology would look like literal magic to someone from any other era of history - that's something worth appreciating.

## Thursday, October 15, 2020

### Emergent monopoles

One of the truly remarkable things about condensed matter physics is the idea that, from a large number of interacting particles that obey comparatively simple rules, there can emerge new objects  (in the sense of having well-defined sets of parameters like mass, charge, etc.) with properties that are not at all obviously related to those of the original constituents.   (One analogy I like:  Think about fans in a sports stadium doing The Wave.  The propagating wave only exists because of the cooperative response of thousands of people, and its spatial extent and propagation speed are not obviously related to the size of individual fans.)

A fairly spectacular example of this occurs in materials called spin ices, insulating materials that have unusual magnetic properties. A prime example is Dy2Ti2O7.  The figure here shows a little snipped of the structure.  The dysprosium atoms (which end up having angular momentum $J = 15/2$, very large as compared to a spin-1/2 electron) sit at the corners of corner-sharing tetrahedra.  It's a bit hard to visualize, but the centers of those tetrahedra form the same spatial pattern as the locations of carbon atoms in a diamond crystal.  Anyway, because of some rather deep physics ("crystal field effects"), the magnetic moments of each Dy are biased to point either radially inward toward or radially outward from the center of the tetrahedron.  Moreover, because of interactions between the magnetic moments, it is energetically favored so that for each tetrahedron, two moments (shown as a little arrows) point inward and two moments point outward.  This is the origin of the "ice" part of the name, since this two-in/two-out rule is the same thing seen in ordinary water ice, where each oxygen atom is coordinated by four hydrogen atoms, two strongly (closer, covalently bound) and two more weakly (farther away, hydrogen bonding).  The spin ice ordering in this material really kicks in at low temperatures, below 1 K.

So, what happens at rather warmer temperatures, say between 2 K and 15 K?  The lowest energy excitations of this system act like magnetic monopoles (!).  Now, except for the fact that electrical charge is quantized, there is no direct evidence for magnetic monopoles (isolated north and south poles that would interact with a Coulomb-like force law) in free space.  In spin ice, though, you can create an effective monopole/antimonopole pair by flipping some moments so that one tetrahedron is 3-out/1-in, and another is 1-out/3-in, as shown at right.  You can "connect" the monopole to the antimonopole by following a line of directed magnetic moments - this is a topological constraint, in the sense that you can see how having multiple m/anti-m pairs could interfere with each other.  This connection is the analog of a Dirac string (where you can think of the m/anti-m pair as opposite ends of an infinitesimally skinny solenoid).

This is all fun to talk about, but is there really evidence for these emergent monopoles?  Yes.  A nice very recent review of the subject is here.  There are a variety of experiments (starting with magnetization and neutron scattering and ending up with more sophisticated measurements like THz optical properties and magnetic flux noise experiments looking at m/anti-m generation and recombination) that show evidence for monopoles and their interactions.  (full disclosure:  I have some thoughts on fun experiments to do in these and related systems.)  It's also possible to make two-dimensional arrays of nanoscale ferromagnets that can mimic these kinds of properties, so-called artificial spin ice.  This kind of emergence, when you can end up with excitations that act like exotic, interacting, topologically constrained (quasi)particles that seemingly don't exist elsewhere, is something that gets missed if one takes a reductionist view of physics.

## Wednesday, October 14, 2020

### Room temperature superconductivity!

As many readers know, the quest for a practical room temperature superconductor has been going on basically ever since Kamerlingh Onnes discovered superconductivity over 100 years ago.  If one could have superconductivity with high critical currents and high critical fields in a material that could readily be made into wires, for example, it would be absolutely transformative to the world.  (Just one example:  we lose 5-10% of generated electricity just in transmission lines due to resistive heating.)

One exotic possibility suggested over 50 years ago by Neil Ashcroft (of textbook fame in addition to his scientific prestige) was that highly compressed metallic hydrogen could be a room temperature superconductor.  The basic ingredients for traditional superconductivity would be a high electronic density of states, light atoms (and hence a high soundspeed for phonon-based pairing), and a strong electron-phonon coupling.

In recent years, there have been striking advances in hydrogen-rich compounds with steadily increasing superconducting transition temperatures, including H2S (here and here) and LaH10 (here and here), all requiring very high (200+ GPa) pressures obtained in diamond anvil cells.  In those cool gadgets, tiny sample volumes are squeezed between the facets of cut gemstone-quality diamonds, and there is a great art in making electronic, optical, and magnetic measurements of samples under extreme pressures.

Today, a new milestone has been reached and published.  Using these tools, the investigators (largely at Rochester) put some carbon, sulphur, and hydrogen containing compounds in the cell, zapped them with a laser to do some in situ chemistry, and measured superconductivity with a transition temperature up to 287.7 K (!) at a pressure of 267 GPa (!!).  The evidence for superconductivity is both a resistive transition to (as near as can be seen) zero resistance, and an onset of diamagnetism (as seen through ac susceptibility).

This is exciting, and a milestone, though of course there are many questions:  What is the actual chemical compound at work here?  How does superconductivity work - is it conventional or more exotic? Is there any pathway to keeping these properties without enormous externally applied pressure?  At the very least, this shows experimentally what people have been saying for a long time, that there is no reason in principle why there couldn't be room temperature (or above) superconductivity.

## Saturday, October 10, 2020

### How fast can sound be in a solid or liquid?

There is a new paper here that argues through dimensional analysis for an upper limit to the speed of sound in solids and liquids (when the atoms bump up against each other).  The authors derive that the maximum speed of sound is, to within numerical factors of order 1, given by $v_{\mathrm{max}}/c = \alpha \sqrt{m_{e}/(2m_{p})}$, where $\alpha$ is the fine structure constant, and $m_{e}$ and $m_{p}$ are the masses of the electron and proton, respectively.  Numerically, that ends up being about 36 km/s.

It's a neat argument, and I agree with the final result, but I actually think there's a more nuanced way to think about this than the approach of the authors.  Sound speed can be derived from some assumptions about continuum elasticity, and is given by $v_{s} = \sqrt{K/\rho}$, where $K$ is the bulk modulus and $\rho$ is the mass density.  Bulk modulus is given by (negative) the inverse fractional change in volume of a substance when the pressure on the substance is increased.  So, a squishy soft substance has a low bulk modulus, because when the pressure goes up, its volume goes down comparatively a lot.

The authors make the statement "It has been ascertained that elastic constants are governed by the density of electromagnetic energy in condensed matter phases."  This is true, but for the bulk modulus I would argue that this is true indirectly, as a consequence of the Pauli principle.  I wrote about something similar previously, explaining why you can't push solids through each other even though the atoms are mostly empty space.  If you try to stuff two atoms into the volume of one atom, it's not the Coulomb repulsion of the electrons that directly stops this from happening.  Rather, the Pauli principle says that cramming those additional electrons into that tiny volume would require the electrons to occupy higher atomic energy levels.  They typical scale of those atomic energy levels is something like a Rydberg, so that establishes one cost of trying to compress solids or liquids; that Rydberg scale of energy is how the authors get to the fine structure constant and the masses of the electron and proton in their result.

I would go further and say that this is really the ultimate limiting factor on sound speed in dense material.  Yes, interatomic chemical bonds are important - as I'd written, they establish why solids deform instead of actually merging when squeezed.  It's energetically cheaper to break or rearrange chemical bonds (on the order of a couple of eV in energy) than to push electrons into higher energy states (several eV or more - real Rydberg scales).

Still, it's a cool idea - that one can do intelligently motivated dimensional analysis and come up with an insight into the maximum possible value of some emergent quantity like sound speed.  (Reminds me of the idea of a conjectured universal bound on diffusion constants for electrons in metals.)

## Thursday, October 08, 2020

### Postdoc opportunities

There is a postdoc opportunity coming up in my lab to look at light emission from molecular-scale plasmonic nanostructures.  It's going to be very cool, looking at (among other things) photon counting statistics (this kind of thing), coupling plasmon-based emission to single fluorophores, all kinds of fun.  Please check it out and share with those who might be interested:  https://jobs.rice.edu/postings/24792

In addition:  The Smalley-Curl Institute is happy to announce that they are accepting applications to 2 (two) J Evans Attwell-Welch Postdoctoral Research Associate positions.   Highly competitive, the Attwell-Welch fellowship was established in 1998 to provide Ph.D. recipients in nanosciences and nanotechnology-related fields, an opportunity to further their basic scientific research experience.

The deadline for the Evans Attwell-Welch submissions is Monday Dec 7th, 2020.  Applications containing the candidate’s resume, a two-page research project, and a letter of support from an SCI member must be emailed to sci@rice.edu before the deadline.  Only applicants sponsored by an SCI Rice faculty member will be considered.

I would be happy to work with a potential applicant, particularly one interested in strongly correlated nanostructures and spin transport in magnetic insulators.  If you're a student finishing up and are interested, please contact me, and if you're a faculty member working with possible candidates, please feel free to point out this opportunity.   Rice University is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability or protected veteran status.

## Saturday, October 03, 2020

### Annual Nobel speculation, + nanoscale views on twitter

It's that annual tradition:  Who do people think will win the Nobel this year in physics?  Or chemistry?  On the physics side, I've repeatedly predicted (incorrectly)  .  Another popular suggestion from years past is Aspect, Zeilinger, and Clauser for Bell's inequality tests.   Speculate away in the comments.

I've also finally taken the plunge and created @NanoscaleViews on twitter.  Hopefully this will help reach a broader audience, even if I don't have the time to fall down the twitter rabbit hole constantly.