Category: Talks/Travel

Anharmonic Phonons are Heating Up

The importance of anharmonicity in the atomic vibrations of crystalline materials has long been known, but the quantities involved have been prohibitively expensive to calculate from first-principles. It is now becoming possible to accurately predict properties including thermal expansion, thermal conductivity, phonon lifetimes, frequency shifts, and displacive phase transitions, for increasingly complex materials.

Last week, I attended a stimulating workshop on anharmonicity in Paris (funded by CECAM) that collected many leaders in the field.  Below is a summary of the selection of currently available techniques and codes. For a primer on the history of the field, I recommend The rise of self-consistent phonon theory by Klein and Horton (1972).

  • AFLOW-AAPL – Automation of phonon calculations and thermal conductivity: [code]; [paper]
  • Alamode – High-order force constants and self-consistent phonons: [code]; [paper]
  • AlmaBTE – Boltzmann transport for device level simulations: [code]; [paper]
  • DynaPhoPy – Anharmonic phonons from molecular dynamics simulations: [code]; [paper]
  • D3q – 3-phonon processes and stochastic self-consistent phonons using random displacements: [code]; [paper]
  • Phono3py – 3-phonon processes and thermal conductivity from finite-displacements: [code]; [paper]
  • SCALID – self-consistent phonon approach, but no longer developed and fails for optic modes: [code]; [paper]
  • ShengBTE – 3-phonon processes and thermal conductivity from finite-displacements: [code]; [paper]
  • TDEP – effective Hamiltonian approach for anharmonic systems from molecular dynamics simulations: [code]; [paper]



Lost in Seoul

(Links updated February 2018) I have had the pleasure to visit South Korea quite a few times now. The people, food and culture have left me with many unforgettable experiences. On our last trip, we made even more friends, enjoyed the most delicious grilled eel, and wandered into Changdeokgung palace just in time for a full-scale reenactment of an 18th century royal birthday party (set after a mad prince died of starvation while being confined in a rice barrel).


This year, I will be back there over the Easter break to teach a course at Yonsei University, which is a good excuse to sort out my random links (and a distraction from learning the basics of Hangul). I am updating these with new discoveries during the trip:

Research Groups
Materials Modelling
Functional Materials

Travel Guides 

Cinema / TV

Transportation / Phone

Vegetarian Friendly Food (listings and reviews)
Hongdae / Sinchon 

Tea / Coffee
Hongdae / Sinchon 

Live Shows

Museums (traditional / modern) (history / art) (abstract Korean art) (modern / contemporary)

They are all naked (how to survive scientific presentations)

I prefer writing to speaking, but from time to time the latter is necessary. When I quite young, I joined a speech and drama club that included mime and poetry. I still remember being on stage at Father Mathew Hall (Dublin City) for a competition. There was no fear in an eight year old me.

That confidence was lost, and I clearly recall my final year presentation as a Chemistry undergraduate, with quivering voice and shaking hands. As a postgraduate, my first conference presentation was at Oxford. I made the mistake of sitting at the back of the room. It was a long and terrifying walk to the front of the auditorium, but I survived. I knew that first talk by heart (and probably sounded like a nervous robot).

Of course presentations play an important role in science, so I had to work at it. I read quite a few books on public speaking (from the dreaded self-help section of the book shop). There were some good general speaker tips:

  • Visit the room early to get used to the presenter view
  • Speak slowly and clearly
  • Face the audience as much as possible
  • Try not to walk around
  • Avoid repetitive words or hand movements
  • Always bring your own pointer and laptop
  • Practice

and some general content tips:

  • Tell a story: a beginning, middle, and end
  • Have a single message per slide
  • Know your audience (tailor the content for them)
  • Use the minimal amount of text
  • Simple is best (no fancy transitions)
  • Large fonts (for the people at the back)

No matter how much you read, it can be difficult to overcome your default behavior and reactions in the moment. Standing in front of your peers is not easy – I have seen senior professors from top universities being as nervous as the first year students. I find it depends as much on the event itself (room layout and setup, audience interest and atmosphere). The worst case scenario is a half empty room of disinterested people with a microphone that keeps clipping. So I find that even with a similar set of slides, a talk can be a disaster, okay or great.

Despite all of the words above, it really comes down to the results. If you have something interesting to present, it doesn’t matter how you do it. One of the most memorable talks I have seen is a student who stood up and delivered a 15 minute monologue (no slides or data). Rules can always be broken.

Help I can’t swim

The absence of regular postings isn’t due to a lack of things to say, simply a lack of free time.

Time management is probably my biggest challenge these days. I have a dozen wonderful group members, doing great research. It is a full time job just to keep up with them, and then I have to find time for my own research, presentations and the bane of all academics… bureaucracy. I am not complaining, I have never enjoyed science as much as I do now.

My research group is now focused on three areas: photovoltaic materials; metal-organic frameworks, and metastable states. Some topics, such as hybrid halide perovskites, are bridging all three themes due to the complexity of their chemistry and physics. A major driving force for our current work is temperature dependent properties (see Jonathan Skelton’s pro-tip guide for Phonopy) and disorder. When I have some free time, I can be found reading some dusty statistical mechanics (one essential read) or thermal physics texts (don’t tell my chemistry colleagues).

Time now to prepare for another trip to my second home (South Korea). This time there is a workshop between Yonsei University and the University of Bath to expand the range and depth of collaborations, followed by an exciting Royal Society collaboration with the Korean Institutes for Basic Science (to be held at Seoul National University). No doubt the next few days will pose exciting adventures in culture, food and functional materials.

8970 km from Bath to Seoul

Last year I wrote about my on-going research collaborations in South Korea. I have been fortunate enough to return twice this year, funded through the Office of Internationalisation at my University. I have just posted a guest blog on the formal delegation activities.

My sincere thanks (again) to the Materials Theory Group of Aloysius Soon. Last week, they treated myself and Adam Jackson incredibly well. We held our first joint research workshop (MICE), which will be followed up in September when four of their group travel to the UK. We also visited KAIST and SNU. Back to the real world now, with a group meeting at 9:15 am this morning!

IMG_6540(Welcome to Yonsei)


IMG_6439(Seoul City Hall)

Materials Overload in Boston

Placing almost 7,000 chemists, physicists and materials scientists in a single space for five days is always going to be an intense experience. The Fall Meeting of the Materials Research Society is one of my favourite conferences, although it is growing so large that it may soon implode. An endless programme of talk titles, over forty symposia, poster sessions that felt like rush hour trains, and a fight for coffee at break-times. The advantage is the variety of science on offer; if you can’t find talks that interest you, then you should probably quit the field!


Boston Bay



The highlight is of course meeting people, but there were also some notable presentations:

(From Symposium Z: Semiconducting Oxides, where I spend most of my time)

Band Lineup of Oxide Semiconductors (Hideo Hosono)
– The world leader of transparent conducting oxides gave a good overview of his group’s recent work; although, band lineups only received a brief mention. The future points towards indirect band gaps.

Band Energies and Doping Limits of Metal Oxides (Aron Walsh)
– Obviously, the talk of the week.

Selection Rule of Preferred Doping Site for n-type Transparent Conducting Oxides (Su-Huai Wei)
– Trying to distinguish between when to dope cation sites (e.g. Al on Zn in ZnO) or anion sites (e.g. F on O in SnO2) for maximum effect.

Thermodynamics of Carrier Compensation in Ga-doped ZnO (Stephan Lany)
– A revision of earlier work on carrier concentrations in ZnO, but with the addition of corrections from GW theory and the contribution of charge-compensated defect clusters.

Temperature Dependence of the Direct Band Gap and Transport Properties of CdO (Tim Veal)
– The band gap of CdO has been revised several times over the past century. A clear picture emerges when band filling and remormalisation effects are treated properly.

(Plenary talks)

Climbing the Ladder of Density Functional Approximations (John Perdew)
– A demonstration that effective presentations do not have to be aesthetic. A powerful recap of the origins of density functional theory and current developments.

Quasicrystals: Discovery, Structure, Properties and Uses (Danny Shechtman)
– “Choose your field and become an expert in it.” An inspiring talk about fighting the establishment with five-fold symmetry.

(From friends who made the week very enjoyable)

P-type Transparent Conducting Oxides: Intrinsic Limitations and Future Directions (David Scanlon)
– A brief summary of his epic PhD work, and an outline of a “madcap” project to screen quinternary oxy-chalcogenides.

MBE Growth and Characterisation of CuCrO2(001) and (015) Thin Films (Russ Egdell)
– How a “simple” material can give a complex reflection when grown on corundum.

Electronic Structure and Surface Properties of Ga and Tl Doped In2O3 (Anna Regoutz)
– Indium may or may not be running out, but it takes guts to investigate Tl doping of indium oxide.

Limits to Doping of Wide Band Gap Oxide Semiconductors and Related Materials (John Buckeridge)
– Re-addressing the doping limits of materials from a chemistry perspective.

Simulation of Hetero- and Nano-structured AgI: The Role of Polytypism and Morphology in Extreme Ag+ Ionic Conductivities (Ben Morgan)
– How ion diffusion is more complex than we commonly think; the small details (really) matter.


Back to the real world now, where today I officially became a Reader (to the confusion of all except UK academics).

Gangnam* Science

This Summer I was lucky enough to have two trips to South Korea. My last trip in 2010 seems like a long time ago. Again, I was impressed by the quality of the science, the food and the people. The visit had three parts:

1. Opening of the Global E3 Institute at Yonsei University
The tradition and formality that comes with the opening of a new institute came as a surprise (white gloves, gold scissors, red carpet). The international links between science, engineering were clear from the start with a mixture of speakers from the US, UK, Korea and Japan. The talks covered everything from post-Fukushima energy policy in Japan to the development of improved thermoelectric devices, and the issues associated with educating children about nano-science and energy materials. I hope that the University of Bath will become part of this consortium in the future, which is being led by Prof. Bob Chang in Northwestern.

2. Seminar at KAIST
The Korean Advanced Institute of Science and Technology is considered the MIT of east Asia. The city of Daejeon is less than an hour from Seoul by fast train, and has a very different feel. The campus is massive, in a beautiful location at the edge of a mountain range. My host was Prof. Yong-Hyun Kim, a former colleague at NREL, and who is now leading the Quantum Nano-Bio Materials Simulation Group. They tackle issues ranging from hydrogen storage and graphene modification to microscopic theories of pH. The graduate school in Nanoscience sets the research bar very high, and I had some very stimulating discussions, as well as delicious food (below is fried kimchee & tofu, along with a special black bean pasta).

3. MTG Tutorial Seminar on Defect Chemistry at Yonsei University
One of the challenges about working on the science of lattice defects in materials is that the description, notation and understanding varies greatly between different disciplines (chemistry, physics and engineering). The group of Prof. Soon is particularly diverse (including students from materials science to civil engineering), so one of the principal goals from such a lecture is to put everyone on the same page with some of the fundamentals. Hopefully it succeeded. Afterwards, we were treated to a very special group Korean barbeque, which I will take as a good sign!

*During a mini-typhoon, I also found some time for shopping in Gangnam at COEX mall (underground, so a safe hideout), visiting a student market in Hongdae, and a little more socialising with the MTG. 감사합니다!