Posts in category Research
Graphene flagship
The ways to finance scientific research in Europe become increasingly creative. The creativity increases certainly faster than that of scientific research. We will have Future and Emerging Technologies Flagships. Six of them have been preselected, two of them will go to high see, four will have to sink directly in dockyard: free competition is free competition. Ships, that is, collections of research groups with common interests and goals, are huge: perhaps only Soviet cold-war-era military research corporations could compete with those.
From the six, the graphene flagship looks most reasonable to me: not from the point of view of a physisist, rather, from that of middle-aged taxpayer. Perhaps I’m too old to believe that science will create Guardian Angels with zero-energy consumption, perform a full computer simulation of human brain and fill the world with helping robots that as soft as teddy bears (I’m not mocking here, I’m citing). Perhaps too young to channel my money into “a revolution in healthcare”. Perhaps too cinical to finance creation of “platform to support decision-making of policy-makers, business people and citizens”. I believe that yellow (web) press does provide such a platform already, and does it without my money.
Jari Kinaret, a Chalmers theorist with recent interests mostly in nanomechanics, will lead the vessel: brave and responsible of him. He came today to Delft to meet Dutch scientists interested in graphene, tell about the ship and listen to us. I’d certainly like this concorcium and wouldn’t be against the boarding the ship.
The only thing is that “boarding the ship” in current circumstances of financial and societal uncertainces remainds me movies of my childhood about events taking place in Crimea in 1920. No I won’t tell (now) about Crimea of that time, since it is not in tone with optimistic meeting of today.
Let her set sails!
Topology and physics: a new conjecture
When I was a student, we were all fascinated by first steps the topological approach made into physics. It took a while to understand that the winding of phase around a vortex is always 2π whatever you do; and that had a sweet taste of intellectual victory. We all read review of Mermin : it was so enlighting to learn that even π in topology has a different, much more profound meaning of homotopy group.
Topology has progressed in years gone, and overwhelms in recent years. There are no more insulators: we’ve topological insulators. They insulate as wet towels, yet are much more profound. The advent of topological superconductors has revolted the field of superconductivity that seemed so stable. And no quantum computing scheme would ever work if topology does not give its protective blessing.
In a kind of conservative rebellition I suggested on Saturday in my talk that perhaps there may be some interesting things in physics that are not based on topology of coordinate space. O boy, how wrong I was. My only consolation is that my wrongness let the truth prevail.
Today Charles Marcus, a Harward professor and most intellectual experimentalist I know, has responded to my talk with a seminal conjecture that I have a great honour to publish.
Marcus’s conjecture
Any result involving integers, including 1 + 1 = 2, can be represented geometrically as a statement of topology, since 1 + 1 = 2 cannot be continuously deformed into any other relation between integers.
Breathtaking. I could have suspected so, why was I so blind…
A note for non-specialist: physics originates from, and is based on counting fingers. For all practical purposes, the result of such counting can be approximated by an integer non-negative number. The conjecture therefore puts physics into the true context: it appears to be a practical exercise in homotopy theory.
A technical note: Charles insisted on presenting his conribution as a conjecture, while the proposition clearly has the status of a theorem. He said he would not present the proof yet. I wonder how he has actually done the proof yet concealed. Being a physist, he could use the traditional medium of this profession, the backside of an envelope. Having understood the importance of topology, he could turn to a margin of a Greek manuscript, the only proper medium for seminal mathematical discoveries since Fermat. Being a conservative rebel, I’m just exploring the consequences of occasional and absolutely unintended dissapearence of this envelop/manuscript. What a wonderful lost for science could it be!
Citation vanity
tears me. Today I’ve recognized that my citation score according to Web of Science has reached 7006. Nice number, isn’t it? Yet nobody has congratulated me…
From conventional to exotic
this is, fortunately or unfortunately, the main direction of scientific research nowadays. This is also the name of a conference organized by Manuel Houzet and Julia Meyer that attempts to summarize recent developments in the field of superconducting hybrid structures, Majorana including.
The conference takes place in magnificient Vercor mountains. Alias no time to go there nor even look around: the talks are too interesting
Tomohiro Yokoyama
from Keio university has arrived today. He will spend three months with us working on spin-orbit interaction in superconducting junctions. Since I’ll depart to a conference tomorrow, he will also go for a vacation, to start with:)
Summary of a discussion session
of the dirty superconductor workshop that was closed yesterday. It’s also posted to the workshop blog. It’s been a marvelous workshop: thank you,Lorentz center and the organizers
Discussion Session I Summary: Quantum Phase slips
The discussion was mostly under the sign of the recent report of Oleg
Astafiev concerning the realization of phase-slip qubit with InO nanowires.
It was noted that a number of features in these devices do not correspond to
usual assumptions about what is good and what is bad for big values of
phase-slip amplitude. Big resistance per square in combination with
significant Tc is certainly thought to be needed for the effect. However,
the issue that required discussion is relatively big width and height of
the wires that by factors exceed coherence length expected for the material.
Another issue concerned the effect of uncompensated charged impurities
readily available in insulating subtrate. The charge induced by these
impurities could lead to a total compensation of the phase-slip magnitude.
It has been noted that a phase-slip in a wire that is wider than coherence
length can be seen as a tunneling of Abrikosov vorthex across the wire. The
fact that the tunneling amplitude is appreciable means that such a vorthex
bears a relatively small energy, perhaps at the scale of quasiparticle
energy, and can be regarded as an elementary excitantion of the dirty
superconductor. An analogy has been drawn with a vortex in Coulomb-blockaded
Josephson junction arrays where a vortex has no core and could have zero
energy. This suggestion is however different from earlier works and
hypotheses concerning dirty superconducting films and Josephson junction
arrays where the possibility of quantum tunneling of already created
Abrikosov vortices has been considered.
Lev Ioffe has outlined his many-body numerical simulations that lead him to
an estimate of phase-slip amplitude in InOx while using a few-site lattice
model. He also suggested that the phase-slip amplitude can be dominated just
by a single optimal path somewhere in the wire, this being in agreement with
other suggestions that weak links can determine the actual magnitude. Such
assumption may aslo explain why the charged impurities do not significantly
reduced the phase-slip amplitude.
It has been noted that the observation of Astafiev is very encouraging for
nanostructuring of the wires to produce more controllable phase-slip devices
where the Coulomb blockade effect can be readily and inambiguosly observed.
The simplest suggestion would be to make two constrictions in the wire
corresponding to two weak links.
It looks like that the report of Astafiev has moved the focus of attention
from the material-science issues to more practical questions. While it
remains to be seen if phase-slips can be observed in traditional candidate
materials like NbSi, TiN, MoGe; InO seems to work. Nano-fabrication and
nano-desing of InO-based devices will probably absorb the attention of
experimentalis for years coming.
Blog dirty superconductor workshop
blogging is a must today, almost the only way to prove somebody’s existence: “Bloggo ergo sum”, as Descartes would put it today. So we in this dirty superconductor workshop also have an extensive blog, here’s the LINK
.
Below is my post to this blog: report on one of the talks.
Vladimir Kravtsov: Electron cooling rate in amorphous films near superconducting-insulating transition
What can we learn from the giant I-V jumps experiments?
The talk presented overwiev of the work made in collaboration with B.L. Altshuler, V.E. Kravtsov, I.V. Lerner, I.L. Aleiner in response for experimental
findings of M. Ovadia, B. Sacépé, and D. Shahar. The experiment has demonstrated a set of hysteretic I-V curves with order-of magnitude jumps and spectacular temperature dependence. It turned out in 2008 that these curves in all details can be explained if electron overheating is taken into account.
An ultimately simple and elegant phenomenological theory is based on a single equation:
IV= joule heating = cooling rate of electrons to phonon bath, and takes as input the linear temperature-dependent resistance R(Te). The speaker outlined the details of the theory demonstrating its sensitivity to the assumptions concerning the temperature dependence of the resistance and cooling rate presenting several simple solution. Further, he concentrated on the coolest part of the story: temperature-dependent electron cooling rate!
He mentioned that the experimental evidence of strong decoupling of electrons and phonons in insulators undermines usual assumptions that the phonon-assistant electron hopping is the dominant transport mechanism in insulators. The temperature dependence extracted from the experimental data clearly demonstrates the rate proportional to T^6 at hight magnetic fields. T^6 law has been derived for common metals yet by Albert Schmid in seventies. It is somehow puzzling that the proportionality coefficient is 2-4-5 orders magnitude larger than the theory of common metal would predict if extended to localized states (the precise number of orders of magnitudes depends on the estimations of sound velocity). The computation of the coefficient for localized states requires more attention but the power law seem to hold: the speaker argued that the fact that the electron states are localized should not by itself lead to Arrhenius law in temperature dependence.
The most dramatic part of the talk concerned the cooling rate extracted from yet
unpublished data at low magnetic fields. The data did display Arrehius law with energy gap of 1.75 K. The speaker argued that this is a clear manifestation of preformed localized electron pairs in the material. He outlined general problems with forming such pairs in insulator if Coulomb interaction is taken into account. He made use of analogy with double-ionization to assure himself and the audience that Nature permits such things.
The talk provoked a discussion that has started slowly but soon become overheated and
involved multiple parties. Sasha Finkelstein has asked a question about phonon-assistant hoping and expressed his surprise with low energy scale invloved
that is in apparent contradiction with Coulomb energy estimations. The blogger wondered why the cooling rate was assumed to be such a simple function of two temperatures. The answer was that this form was obtained yet by Schmid but eventually
has no apparent reason to be general. Misha Gershezon has shared his experience in measuring colling rates and posed a series of questions addressed to experimentalists and concerned with time scales of cooling. Zvi Ovadyahu mentioned that overheating bistability is readily observed at room temperature. Why does one have to go to low temperatures? The answer: to get cooling rate at low temperatures.
The discussions in groups have lasted at least half an hour after the talk.
Phase-slip qubit realized?
I’m in Leiden this week, participating in a workshop “Strongly Disordered Superconductors and Electronic Segregation” organized by Mikhail Feigel’man and Teun Klapwijk.
The main event of today (my seminal talk excluding:) ) was a sensational report of Oleg Astafiev of NEC. He has outlined his recent measurements on devices where InO was playing the role of a disordered superconductor. He made use of a desing proposed bij Hans Mooij and Kees Harmans about six years ago, so called phase-slip qubit. And the observation seems to show a well-developed lifted anticrossing of flux states corresponding to the phase-slip amplitude of more than 4 GHz.
As any pioneering experiment, this one rises some questions, especially around the numbers involved. However, it all looks real, for me at least.
Quantum phase slips in superconducting wires with weak links
is the topic of another cond-mat submission today, please follow the
LINK. This is the outcome of post-doc term of Mihajlo Vanevic, who worked two last days voluntary to accomplish this submission. It was a hard project, results of which are more interesting, and – forgive me this expression – elegant then we expected in the beginning. Mihajlo does not want to stay in West anymore: he gets reseach position in Belgrado.
Here’s the abstract:
Quantum phase slips are traditionally considered in homogeneous diffusive wires. We argue that even in realistic, apparently homogeneous wires, the phase slips can occur at weak links where the local resistivity is slightly higher. We model the weak link as a general coherent conductor and obtain accurate estimate of the quantum phase-slip amplitude. We speculate on a better estimation of the amplitude for homogeneous wires as well.
Re-flows on arxiv
,finally. I’m quite proud of and excited by the paper. The excitement was so high that I’ve been working yesterday the whole day on the text, fixing small things, but, quite embarassingly, have completely forgotten to run it through a spell-checker… Oh.
Here’s the LINK.
The abstract is rather long:
We demonstrate that the condensed matter quantum systems encompassing two
reservoirs connected by a junction permit a natural definition of flows of
conserved measures, Renyi entropies. Such flows are similar to the flows of
physical conserved quantities such as charge and energy. We develop a
perturbation technique that permits efficient computation of Renyi entropy
flows and analyze second- and fourth order contributions. Second-order
approximation was shown to correspond directly to the transition events in the
system and thereby to posess a set of “intuitive” features. The analysis of
fourth-order corrections reveals a more complicated picture: the “intuitive”
relations do not hold anymore, and the corrections exhibit divergencies in
low-temperature limit manifesting an intriguing non-analytical dependence of
the flows on coupling strength in the limit of weak couplings and vanishing
temperatures.