Sunday, August 19, 2018

Big changes in universities?

I was recently asked to give a talk to an NGO about how universities are changing.
There is no doubt that there are rapid changes, many for the worse, happening. For me the biggest change has been the rising influence of neoliberalism (free market ideology) in the values, goals, and decision making within universities. But that is another story...
In the past few weeks some relevant articles "came across my desk" [through my web browser...].

I would be particularly interested to hear readers comments on the first one.
My wife sent me this New York Times piece to read and I really despaired
The iGen Shift: Colleges Are Changing to Reach the Next Generation 
The newest students are transforming the way schools serve and educate them, including sending presidents and deans to Instagram and Twitter.

Why do I despair?
I believe a university education should largely be about two things. The first goal is explicit and the second is implicit.

The first and primary goal of university education is to help students to learn to think: to think about specific disciplines, and to think about anything. Deep and valuable thinking does not come easily. It takes time, concentration, perseverance, and freedom from distractions. It a student is constantly checking their phone or just skimming the first material that comes up on a Google search or in the Twitter (for twits!) feed for their course, it is highly unlikely they are going to do much deep thinking.

Second, a university education is about helping students "grow up":  to move towards adulthood, to learn to be responsible and independent, to create an identity that is more independent of their parents and their peers, to have a sense of direction and purpose, and a desire to be good citizens.
Unfortunately, too many students think "growing up" means getting really drunk and throwing up...
However, too many of the initiatives described in the article seem to be pandering to students (customers) and "hand holding" them through their university experience. For example, at some point in life students are going to have to learn to seek and find relevant information, regardless of whether it is nicely packaged in some cool app with great graphics and lots of "likes".

Oh, the Humanities! 
New data on college majors confirms an old trend. Technocracy is crushing the life out of humanism.
Ross Douhat (NYT op-ed columnist)

The third article is more controversial and political, and raises questions about whether one purported change is as big or significant as often claimed (particularly in the right-wing press)
The free speech panic: how the right concocted a crisis
Snowflake students have become the target of a new rightwing crusade. But exaggerated claims of censorship reveal a deeper anxiety at the core of modern conservatism.
William Davies (A Long Read in The Guardian)
For balance here is a counter-view that there is a problem
The Problem of Hyper-liberalism
John Gray (Times Literary Supplement)

Comments welcome.

Wednesday, August 15, 2018

Solid State or Condensed Matter Physics?

The two terms are often used interchangeably, but that is not appropriate. Condensed matter physics does not just involve solids but also phenomena in liquids, liquid crystals, superfluids, and polymer melts.  Solid state physics is a subset of condensed matter physics. The latter term was arguably coined by Phil Anderson, when he and Mott renamed their research group at Cambridge in the 1970s. One can view research fields or course titles as a list of topics or as a way of thinking about certain parts of reality. Solids exhibit rich phenomena including magnetism and superconductivity. However, it is best to actually view the solids as (an almost irrelevant) substrate for the phenomena.

Like many things, this perspective arguably started with Landau. His theory of phase transitions in the 1930s did not consider atomic structure or chemical composition. Even structural phase transitions were viewed in terms of symmetry change, not in terms of explicit microscopic details. In 1950 this led to the Ginzburg-Landau theory of superconductivity. This all suggested a unified approach to phase transitions.
Furthermore, Landau's Fermi liquid theory papers were originally concerned with understanding liquid 3He, not electrons in metallic crystals.

This idea was further highlighted in the 1970s with the study of critical phenomena and the associated idea of universality. Specifically, the critical behaviour of an XY magnet, a superconductor, and a superfluid, are the same (i.e. they have the same critical exponents). The critical behaviour of the liquid-gas transition, an Ising magnet, and the order-disorder transition in a binary alloy are the same. The view that the solid state might actually not be the key feature for understanding and describing superconductivity was highlighted in the 1950s by Fritz London in his two-volume book, Superfluids, which suggested the two phenomena were intimately connected. Beginning in 1968, De Gennes took a condensed matter perspective in applying order parameters and scaling ideas to “soft matter”: liquid crystals, polymers, wetting, …

The important element to this conceptual view of condensed matter is that it provides a unifying perspective on phenomena in a diverse range of materials. It also brings to the fore how a wide suite of powerful theoretical and experimental tools (esp. neutron and x-ray scattering) can be used to study diverse materials. One of the key theoretical strategies is that of effective Hamiltonians, which is not unique to condensed matter, because it just reflects the hierarchy of energy, length, and times scales that result from emergence. This then leads to an intellectually rich interchange of ideas and techniques from other fields of physics, particularly quantum field theory.

More recently, this unity is illustrated by ultracold atomic gases which can be used to study some phenomena that had previously only been studied in solids.

Saturday, August 11, 2018

Hype, DNA, drugs, and emergence

Unfortunately, hype in science reflects hype in broader society, including in business. The complete DNA sequencing of the human genome was an amazing scientific achievement. Unfortunately, it was also associated with a lot of hype about what this would mean for medicine and for the pharmaceutical industry. This issue is made painfully and succinctly in a recent column in the business section of  The Guardian by Nils Pratley.
It has been almost two decades since the first bosses of the newly merged GlaxoSmithKline talked up the medical wonders that would flow from the unravelling of the human genome. GSK would become the “Microsoft of the pharmaceutical industry”, they said.  
To put it mildly, the corporate vision hasn’t been realised. GSK’s share price stood at £20 at the time of the turn-of-the-century merger and is £15.42 today. Lack of productivity in the labs has been a constant complaint. The genetics revolution is happening, but not at the pace originally promised, at least not at GSK.
These challenges could have been forseen by filtering the hype through a emergentist perspective, such as that presented beautifully by Denis Noble in a 2006 book, The Music of Life: Biology beyond the Genome.  Knowing a DNA sequence is about as useful, for better or worse, as knowing the many-body Schrodinger equation for a plutonium crystal. A great place to start, but ....

Thursday, August 9, 2018

Emergent temperature scales and spin-orbital separation in the Hund's metal

An important and fascinating issue in many-body physics is the emergence of new energy scales, particularly scales that are orders of magnitude smaller than the energy scales in the underlying Hamiltonian. One example is the coherence temperature associated with the crossover from a Fermi liquid (with coherent quasi-particles) to a bad metal.

Recently, I posted about the crossover from a Hund's metal to a bad metal, seen in the collapse of the Drude peak in the optical conductivity, and the issue of capturing this slave-particle theories. One commenter mentioned the relevance of the paper below and another asked about the claim that the Kondo effect is associated with the collapse.

I agree that Kondo physics is associated with the crossover. Although, far from obvious this is also the case in the single-band Hubbard model. The Kondo effect was first studied with isolated magnetic impurities in metals and can be described by a single-impurity Anderson model (SIAM). Although there are no magnetic impurities in the Hubbard model, it turns out that when studied at the level of Dynamical-Mean-Field Theory (DMFT), the model is described by a self-consistent SIAM and close to the Mott metal-insulator transition Kondo physics does emerge. Specifically, the Kondo temperature for the self-consistent SIAM corresponds to the temperature at which there is a crossover from local unscreened local magnetic moments (associated with the almost-localised electrons near the Mott phase; the bad metal) to a Fermi liquid where the "magnetic moments" are screened.

What happens in a two-band Hubbard-Kanamori model with Hund's rule coupling?
The physics is richer because there is now the possibility screening of spin and/or orbital degrees of freedom, and of a orbital-selective Mott phase (or bad metal). 
This is nicely investigated in the following paper.

Dynamical Mean-Field Theory Plus Numerical Renormalization-Group Study of Spin-Orbital Separation in a Three-Band Hund Metal
K. M. Stadler, Z. P. Yin, J. von Delft, G. Kotliar, and A. Weichselbaum

For me, the figure below is the most interesting and illuminating. It shows how due to the Hund's rule coupling, two distinct energy scales (differing by about two orders of magnitude) emerge and associated with screening the spin and orbital degrees of freedom, respectively.

This is Kondo physics, but there are no magnetic impurties.

Tuesday, August 7, 2018

Philosophy and emergence in condensed matter

Condensed matter physics is a source of a multitude of beautiful examples of emergence.  On the other hand, for more than a century philosophers have thought seriously about emergence, partly motivated by profound and difficult questions concerning human consciousness and free will.
Prior to the past decade, there appear to have been no substantial interactions between physicists and philosophers about the subject. A few years ago I posted about some recent work by philosophers of science on quasi-particles.

One of the big issues that philosophers wrestle with is the relative merits of weak emergence and strong emergence, which are sometimes distinguished as epistemological and ontological emergence.

I am very happy that in the past year or so that philosophy journals have published more than half a dozen papers about emergence in condensed matter. One of the papers, by Stephen Blundell, I blogged about earlier. Here I will mention two others and discuss one. All the papers are a result of the Durham Emergence Project.

Strong emergence and downward causation in biological physics
Tom C. B. McLeish

Reduction and emergence in the fractional quantum Hall state 
Tom Lancaster and Mark Pexton

McLeish begins with a helpful and succinct summary of the argument by Jaegwon Kim about "the causal completeness of the physical" [or the argument against non-reductive physicalism] that leads to the conclusion that mental events cannot have physical consequences. This argument has attracted significant attention from philosophers and has been used against strong emergence, and particularly to argue that consciousness is reducible.

McLeish rightly points out that the problem of consciousness is a "can of worms" [my phrase] and instead it might be valuable to consider the issue of "downward causation" by considering three important examples in biological physics.

"Downward causation" means "there are high-level entities, carrying unique information about the system essential for its future evolution, and whose form and evolution are not determined entirely by the low level entities."
He gives a nice introduction to soft matter physics and its applications to biological systems, considering the following examples.
  • Membrane and intra-membrane self-assembly
  • Allosteric Signalling in Gene Expression
  • Entangled DNA and Topoisomerases  
He points out how in these systems there is "top down causation" and that the emergent entities such as protein elasticity are not just a result of "coarse graining" but new "long-range physics" that arises from many microscopic realisations.

McLeish considers these examples reflect what Bishop and Silberstein (2016) defines as ``‘epistemological contextual emergence’ (ECE) as applying to systems whose ...description at a particular descriptive level (including its laws) offers some necessary but no sufficient conditions to derive the description of properties at a higher level.''

I thank Stephen Blundell and Tom McLeish for helpful discussions about their papers.

Thursday, August 2, 2018

Phase diagram of snowflakes

I like "collecting" interesting phase diagrams, partly because they are fun to show students when teaching introductory thermodynamics. I recently discovered the one below that I feel I really should have known about. It shows the morphology of different snow crystals as a function of temperature and water supersaturation (relative to ice).
It should be pointed out that this is a non-equilibrium phase diagram as it involves supercooled liquid water.

The figure below is taken from the beautiful review
The physics of snow crystals 
Kenneth G Libbrecht

This diagram was originally constructed by Ukichiro Nakaya in the 1930's. The physics behind it is still poorly understood.

I came across the diagram while browsing through the Forces of Nature book by Brian Cox and Andrew Cohen.

While on the subject here is a nice video.

Monday, July 30, 2018

Experimental observation of the Hund's metal to bad metal crossover

A definitive experimental signature of the crossover from a Fermi liquid metal to a bad metal is the disappearance of a Drude peak in the optical conductivity. In single band systems this occurs in proximity to a Mott insulator and is particularly clearly seen in organic charge transfer salts and is nicely captured by Dynamical Mean-Field Theory (DMFT).

An important question concerning multi-band systems with Hund's rule coupling, such as iron-based superconductors, is whether there is a similar collapse of the Drude peak. This is clearly seen in one material in a recent paper

Observation of an emergent coherent state in the iron-based superconductor KFe2As2 
Run Yang, Zhiping Yin, Yilin Wang, Yaomin Dai, Hu Miao, Bing Xu, Xianggang Qiu, and Christopher C. Homes

Note how as the temperature increases from 15 K to 200 K that the Drude peak collapses. 
The authors give a detailed analysis of the shifts in spectral weight with varying temperature by fitting the optical conductivity (and reflectivity from which it is derived) at each temperature to a model consisting of three Drude peaks and two Lorentzian peaks. Note this involves twelve parameters and so one should always worry about the elephants trunk wiggling.
On the other hand, they do the fit without the third peak, which is of the greatest interest as it is the sharpest and most temperature dependent, and claim it cannot describe the data.

The authors also perform DFT+DMFT calculations of the one-electron spectral function (but not the optical conductivity) and find it does give a coherent-incoherent crossover consistent with the experiment. However, the variation in quasi-particle weight with temperature is relatively small.