Science

New papers

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I’m on a few new papers that have appeared this year. If anyone reading this is interested in these topics, contact me.

Determining the site preference of trivalent dopants in bixbyite sesquioxides by atomic-scale simulations
by CR Stanek, KJ McClellan, BP Uberuaga, KE Sickafus, MR Levy and RW Grimes
Physical Review B 75, 134101 (2007)

Abstract: Oxides with the bixbyite structure have two crystallographically unique cation sites, namely (in Wyckoff notation) 24d and 8b. Since the symmetries of these two sites are different (C2 and S6, respectively), properties related to solute cations will vary depending on the site preference. Therefore, we have employed atomic scale simulation techniques to systematically investigate the solution site preference of a range of trivalent cations ranging from Sc3+ to La3+ in A2O3 bixbyite oxides (where A ranges from Sc to La). Results reveal that when the solute cation is smaller than the host lattice cation, the 24d site is energetically favorable, but when the solute cation is larger than the host lattice cation, the 8b site is preferred. We also discuss the tendency for solute cations to cluster, as well as corroboration of this work by first principles methods.

Defect kinetics in spinels: Long-time simulations of MgAl2O4, MgGa2O4, and MgIn2O4
by BP Uberuaga, D Bacorisen, R Smith, JA Ball, RW Grimes, AF Voter and KE Sickafus
Physical Review B 75, 104116 (2007)

Abstract: Building upon work in which we examined defect production and stability in spinels, we now turn to defect kinetics. Using temperature accelerated dynamics (TAD), we characterize the kinetics of defects in three spinel oxides: magnesium aluminate MgAl2O4, magnesium gallate MgGa2O4, and magnesium indate MgIn2O4. These materials have varying tendencies to disorder on the cation sublattices. In order to understand chemical composition effects, we first examine defect kinetics in perfectly ordered, or normal, spinels, focusing on point defects on each sublattice. We then examine the role that cation disorder has on defect mobility. Using TAD, we find that disorder creates local environments which strongly trap point defects, effectively reducing their mobility. We explore the consequences of this trapping via kinetic Monte Carlo (KMC) simulations on the oxygen vacancy (VO) in MgGa2O4, finding that VO mobility is directly related to the degree of inversion in the system.

Parallel replica dynamics for driven systems: Derivation and application to strained nanotubes
by BP Uberuaga, SJ Stuart and AF Voter
Physical Review B 75, 014301 (2007)

Abstract: We show that parallel replica dynamics can be extended to driven systems (e.g., systems with time-dependent boundary conditions). Each processor simulates a replica at a driving rate that is M times faster than the desired rate, where M is the number of processors. As in regular parallel replica dynamics, when a transition to a new state is detected on any processor, the times are summed and every processor is restarted in the new state. The state-to-state dynamics are shown to be correct if the processors run at the same speed and the system is driven slowly enough (on each processor) so that the escape rates do not depend on the time history of the drive. We demonstrate the algorithm by stretching a carbon nanotube with a preexisting vacancy, noting a significant dependence of the nature of nanotube yield on the strain rate. In particular, we are able to achieve strain rates slow enough such that the time scale for vacancy diffusion is faster than that for mechanical yield at a temperature of 2000 K. We thus observe vacancy-induced morphological changes in the nanotube structure, providing some insight into previously unexplained experimental features.

Rant

What is power?

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While in London, a few of us found ourselves in a pub, chatting over a few pints. We had a really interesting discussion about the British monarchy and the powers of the queen. (Many interesting discussions do seem to occur in pubs.)

(As an aside, the photo shows the Crown Jewels of Britain. The big red stone is known as the Black Prince’s Ruby. In reality, it is not a ruby but a spinel, a mineral we study because of its radiation tolerance and potential applications in nuclear reactors and fusion tokamaks.)

The main jist of the conversation was what powers the queen had. It seems that she is the ultimate authority in Britain, with powers that trump even those of the prime minister. At least, this is what is in the law and there is no disputing this.

What was in dispute, at least by me, is if these powers are “real”. I mean real in the sense of can the queen use these powers? In theory, the answer is definitely yes. But in practice, can she really? It seems to me that if she ever tried to use the more powerful of the powers she has (as, for example, commander in chief of the military), a number of things might happen. Most Brits, I believe, feel they live in a democracy. As such, I think they expect that their elected officials make the decisions that affect the country (even the foolish ones like supporting Bush in Iraq). I think that these people, who nominally support the existence of the queen, would be pretty irked if she used her powers to over turn any decision made by parliament. It might be the last straw for a lot of people who would then demand the end of the monarchy.

It thus seems to me that the queen, while in reality having such powers, can’t use them as they might lead to a “revolution” and/or the end of the monarchy. It thus strikes me that her powers are really useless and she has no power at all.

Of course, the Brits at the table argued the opposite, which I found somewhat amusing. They all seemed to feel that if the queen did such an act, did something completely against what parliament decided, that the people would merrily support her actions. They felt that the queen was a check against the parliament and the self-interest of elected officials. They thought the House of Lords served a similar purpose.

I can’t really understand this perspective. It seems that the basis for it is that the common man, and the people they elect to office, are dolts and need someone more enlightened to make sure they don’t get out of hand. This is where the nobility comes in. But, to me, these people have no authority other than the fact that their ancestors possibly hundreds of years earlier had gained power through either money or bravery on some battlefield. While this might have meant some of them were “worthy” of positions of influence, certainly their descendants have no such claims. I might see it as a decent idea if the people in these positions had gotten their based on merit, if they were the best scientists, business people, artists, etc that the country had to offer. But, they aren’t. They are just the ones that were born into some special office. I’ll never understand how the Brits can happily live in such a system.

In the end, we never reached any consensus or changed any minds. Maybe I got to understand their mind-set just a bit more, but I’m not sure. It is a bit too foreign to me. I prefer our system, even if it leads to a situation where someone like Bush is running amok in the world. While there is no check on him (other than Congress and the Supreme Court, though those have been less than willing to do their duties recently), the ultimate responsibility of his actions comes back to the people that elected him. Thus, we, the American people, have to bear that responsibility, learn from our mistakes, and do a better job in the future.

Travel

An “accelerated” trip to England

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I just got back from a one-week trip to the UK for a work trip. This is the shortest trip I’ve done overseas. Usually, I like to go for two weeks, just to make the adjustment to the timezone worth it. But, as I’ve got too many things to do back home, I kept this one short. In the end, I think it will be fine. While I had a struggle at times staying awake, I made it through.

The purpose of the trip was to attend and present at a workshop on accelerated molecular dynamics and long time scale simulations. That was the subject of my postdoc at Los Alamos. The idea was to bring together many of the people involved in this research and give the ideas greater exposure in the UK. Most of the talks were very interesting. There were a lot of neat new ideas presented. It was hosted at Loughborough University, in north/central England. They have a wonderful campus, one of the largest, in terms of land, in the UK.

Our host took us to a traditional British pub which served traditional British food. I stayed away from the “typical” fare (black pudding, for example) and got Steak and Guinness pie, which was very good, with no vegetables and extra mashed potatoes.

After Loughborough, I headed to London to spend a day with a collaborator at Imperial College. When I arrived, he and his students took me to a Lebanese place which was quite good. We had a number of starters, all vegetarian, and then gorged on lamb and chicken for the main course. It was very well prepared and tasted great. We spent the next day talking about various projects of common interest. In the evening, my host took me to his place for a BBQ, which he lugged all the way from America after his sabbatical at Los Alamos.

Overall, the trip, while fast, was good. A lot of time spent traveling. I flew in via Atlanta and Amsterdam to Birmingham, as that is only about 30-40 miles from Loughborough. Then I took the train to London. The flight over was a bit obnoxious as they didn’t have the personal TV for each seat. I don’t even remember what we watched. I did a lot of reading of papers and stuff. I took three books and didn’t crack a one. I guess I was too tired to read for enjoyment.

On the way back, I did have one of those fancy personal TVs with my seat. That was cool. I watched an episode of House, which is always entertaining. I saw the first episode of Heroes and was pleasantly surprised at how well I liked it. I also saw Casino Royal (I’m a bit confused by the ending, but will wait to comment on that until I watch it again with Lisa, since we have it at home waiting from NetFlix) and Hollywoodland, which I vaguely remember hearing about but didn’t know anything about. That was good. It has Ben Afleck, who I could give or take, but Adrian Brody was also in it and he seems like a good actor. It is one of those morally ambiguous tales. I highly recommend it.

Also with the TV was an onboard trivia game that you can play against other passengers, which was cool. I won one of the maybe 5 rounds I played. But, there was one question that annoyed me: categorized as “Science and Technology” they asked: what sign would someone born on October 2 be? What does this have to do with “Science and Technology”? It would be fine as “General Knowledge”, but it just does not belong in the “S&T” category. There is no science nor technology in astrology.

Rant, Science

Speaking of hypocrisy… the Goracle

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Some might infer from my last Rant that I have something against conservatives, singling them out for my vitriol. Well, while I do tend more toward the Democratic side, I don’t think hypocrisy is the sole purvue of the Republicans.

In a recent issue of Time magazine, Charles Krauthammer writes about Al Gore and his campaign to make the world recognize the dangers of global warming. While I believe that this is an important campaign and that we, the global we, must do something about it, Krauthammer makes a number of good points about Gore’s approach, some of which I’d had issues with myself.

The basic problem with Gore (or as Krauthammer calls him, the Goracle) and his message is that he doesn’t practice what he preaches. His mansion in Tennessee consumes 20 times as much power as the average home in the US. That is, of course, enough to power 20 of Gore’s neighbors (well, probably not Gore’s neighbors, but still). And yet, Gore is “carbon neutral”. He gets this status by purchasing carbon credits, which means that he pays to have some other place reduce their carbon emissions by either reducing their amount of pollution or planting the corresponding number of trees to soak up the equivalent amount of carbon dioxide. Usually, these “other places” are third world countries.

I’ve had issues with the idea of “carbon credits” since I first heard of them. They don’t solve the problem of global warming. If we don’t have caps, carbon credits may lead to more trees being planted in Africa, but it doesn’t stop the construction of new coal burning plants next door to the newly planted trees — the overall carbon emissions don’t necessarily decrease and may just as likely go up. If we do have caps, eventually the rich countries will buy all of the carbon emission rights, leaving none for the poor countries who are then unable to develop their economies. This may seem to be not much of a problem for the rich countries, but it would exacerbate resentment of poor countries towards rich countries and problems like illegal immigration. If the overall goal is to get all of the world’s countries to roughly the same standard of living, carbon credits with caps is definitely not the way to go.

To solve the problems of global warming will take a dramatic shift in how we live. It is impossible, at this time, to imagine our lives without any carbon emissions, and I don’t think I or any sensible person is advocating that. But, it will require that we change how we live.

I don’t claim to have done much in this area myself. I try to do the little things: I carpool to work, removing two cars off the road; my wife Lisa and I bought a relatively fuel-efficient car (a Ford Focus); we don’t have multiple TVs blaring at the same time (though during football season, I do have a second one going to watch the games); we have a high-efficiency washer and dryer. But, these aren’t radical steps.

However, I’m not claiming to be carbon neutral. This is the problem I have with Gore. While I believe in his message, that we need to do something now to solve global warming, I don’t believe in the messenger. When his idea of being carbon neutral isn’t to change how he lives but to buy carbon credits, he loses all credibility with me. And I’m on his side! How does the other side, the ones that are skeptical of global warming, view him and his message?

The average person cannot afford to do what Gore is doing. For Gore, paying for some carbon credits is no big deal as he has the wealth to afford it. So does most of Hollywood, who lavished him with so much praise recently. The average American cannot afford these credits. So, even in the US, eventually you will have the rich who can pollute all they want while the poor are effectively paying for it by having to make the changes in their lives to compensate. And by poor, I mean in a relative sense; the middle class will have to pay.

No one likes a messenger who doesn’t live by his message. How well would Jesus’ message had been received if, while accosting the money lenders in the temple, he was a leader of a crime gang? If the Apostles were out roughing people up in the streets for money? Jesus’ message, Ghandi’s message, Martin Luther King’s message, all gathered strong adherents and admirers because there is a sense that they lived their message. They didn’t just preach their message, they were their message.

If Gore is going to convince the vast majority of Americans that global warming is serious and serious steps need to be made to curtail it, he needs to change the way he lives. Otherwise, he will always be a target for his critics who just write him off as a hypocrit.

Science

Mathematical Proof for Diversity

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Last week, Lisa and I joined some friends for a lecture put on by the Santa Fe Institute. SFI is known for its quirkiness, and this was the first of their lectures I had attended, so I didn’t know quite what to expect. The speaker was Scott Page, who is at SFI but also at Michigan. He is a computational economist, using methods like agent-based modeling to study economics and how they impact societies.

Page spoke about diversity and how it plays a role in problem solving and prediction. However, he approached the topic from a mathematical perspective. He began with some anecdotes which “demonstrate” that diversity is good for problem solving, predicting, etc. He told of one story, in particular, from an old English fair in which people were trying to guess the weight of a steer. The average guess was withing 1 pound of the right answer! These kinds of observations are what has led to the formal study of diversity and the role it plays in groups.

The short answer is that diversity often helps. If all of the people in a group are “smart” relative to a problem (that is, they know something about the topic; they are not completely ignorant about it, like a non-mechanic trying to fix a car), then it is better to have a diverse group, including what Page called the “pinhead”, rather than a whole bunch of people who are all the very best, but are all similar. Invariably, in computer models, the diverse group always solves problems better than the “better” group. This is because they have more tools at their disposal, as the pinhead has some tools the genius does not have. By working together, they can solve a wider range of problems than if the group only had geniuses.

Related to this is the predictive quality, and there is actually a mathematical equation relating the error a crowd makes in predictions to the diversity of the crowd. If the diversity is greater, the average prediction of the crowd has a smaller error. This has been demonstrated by looking at expert predictions for, for example, sports (NFL, NBA) drafts, comparing each expert’s pick with the average guess. In almost all cases, each individual expert had more error in their predictions than did the average prediction.

I asked Page about cases where diversity hurts and he pointed out that irreversible processes, such as cooking, are cases were diversity hurts. If I throw chili peppers into the soup, it doesn’t matter what tools you have, you can’t undo what I did. If my peppers ruined the soup, it is ruined no matter how many people are helping. The military, he pointed out, is an interesting case: you want diversity in planning, to come up with the best plan, but you don’t in operations, as you want people to follow the plan already made up. They need to be more single-minded in operations.

Thus, the two adages: “Two heads are better than one” and “Too many cooks spoil the stew” are both right. It is only now, though, that math and science can begin to tell us under which conditions one or the other applies. This is fascinating stuff!

Blah, blah, blah… I've got the blahs.