The third Thursday crept up on me this month, arriving only 15 days
into September, so for those of you on the African side of the world,
it's Friday by now. I hope you are all doing well, and that you've
had a chance to use some of the things you learned at the workshop.
Next month I'll be traveling when the third Thursday rolls around,
so Sinead will take over the newsletter.
:: CONTENTS ::
1) Long-distance help with calculations
2) Free journal subscriptions and the arXiv
3) Article of the month: Calculating Band Offsets in Heterojunctions
4) Group-project update
:: LONG-DISTANCE HELP WITH CALCULATIONS ::
Since the workshop, we mentors have been delighted to receive emails
from people asking for help with their Quantum Espresso
calculations. We want our interactions to be as beneficial as
possible, and you would probably like to get help quickly without
waiting for several days of back-and-forth email with someone in a
different time zone. Toward that end, here are a few things to keep
in mind when emailing us about problems with your calculations.
1) Send all relevant information. In order to figure out what's
going on and help trouble-shoot the calculation, we typically need
to see all the input and output files, the job script (if you are
using one), and any errors written to standard out (on the screen).
Often it will be easiest to make a tarball so you just have one
attachment. (gtar -zcvf <foo.tgz> <file1> <file2> ....)
2) Carefully describe your problem. What exactly are you trying to
achieve with the calculation? What have you tried so far? Very
often I find that the process of writing a careful email actually
helps me figure out a problem for myself, as it forces me to slow
down and really consider what's going on.
3) Provide feedback after receiving help. Did our suggestions fix
the problem? Did you figure out a different solution? This gives
us a chance to learn from you, too, and ensures that we don't keep
working on something you've already solved.
4) Consider cc'ing more than one mentor on your email. That way,
you can get help from the first person available instead of waiting
for a response from someone who may be in the middle of writing a
proposal. You also tap into a bigger pool of knowledge, in case one
of us has seen a similar problem before.
Good luck with your calculations, and keep the questions coming!
:: JOURNAL ACCESS WITHOUT SUBSCRIPTIONS ::
Last month I promised that the September newsletter would include
information about how to get research articles if you can't afford a
journal subscription. Here I'll share with you the little I know or
have been told, but I invite people to send me their tips as well.
Eventually I will compile all our combined knowledge on the
The first suggestion, which Prof. Martin told us about at the
workshop, is to request special access directly from the journal.
Apparently, the American Physical Society journals, like Phys. Rev.
Lett. and Phys. Rev. B., will provide free subscriptions to
universities in Africa. In order to get a subscription, a librarian
from the University must contact the journal and request it, so you
may need to get this process started yourself. I've yet to learn if
other journals have a similar policy, but will investigate this.
Another option is to look on the physics arXiv, which is an on-line
repository of articles in physics, math, and computer science.
There isn't a peer-review process for the arXiv, but many authors in
condensed matter physics put their journal articles on there even
before they are published. (http://arxiv.org. By the way, the name
is an awesome joke for physicists: Because we are familiar with the
Greek letter Chi, which looks like an 'X', we know that the word
"arXiv" is actually pronounced "archive". If you just laughed, you
are probably a nerd.)
If you have other suggestions for gaining access to journal articles
from Africa, please send me an email, write a comment on the
website, or start a discussion on the forum.
:: RESEARCH ARTICLE OF THE MONTH ::
From Kris Delaney:
"Band Offsets in Lattice-Matched Heterojunctions: A Model and
First-Principles Calculations for GaAs/AlAs", A. Baldereschi, S.
Baroni and R. Resta, Phys. Rev. Lett. 61, 734 (1988).
This classic article introduced what has since become the standard
method for computing band offsets at planar interfaces using
first-principles electronic structure methods. When two dissimilar
semiconducting/insulating materials are joined at a planar
interface, the electrical behavior is largely determined by the
alignment of valence and conductions bands at the interface. This
is a well-known situation that occurs in all semiconductor devices,
ranging from CPUs to semiconductor lasers to thin-film photovoltaic
cells, and its accurate description is crucial to performing useful
Howeer, electronic-structure calculations of bulk (infinite)
crystals have no absolute reference energy, meaning the band
energies can be arbitrarily shifted without changing and physical
properties. This means that the only way to compute band offsets is
to explicitly perform a calculation with a supercell containing the
interface. (Surprisingly, despite the breaking of periodicity and
the long-range nature of the Coulomb interaction, one can reliably
compute the band alignment of arbitrarily thick slabs using a
supercell with very few layers of each material.)
Obtaining the band offsets from an electronic-structure calculation
of a supercell is simply a matter of tracing some suitable reference
energy across the interface, but these references usually contain
spatial oscillations from the underlying crystal structure. This
paper introduces the macroscopic averaging technique, which is a
reliable method to "smooth out" spatial oscillations in reference
levels, leading to accurate and unambiguous calculations of
interface band alignments.
:: GROUP PROJECT UPDATE ::
The group project from the workshop is still underway! We are
working to put together a summary of the progress, to be sent by
email or posted on-line, so keep an eye on the website announcements