Paring with spin fluctuations

A review of an interesting work observing the paring mechanism of an exotic superconductor.

Hattori et al. are able to correlate this field-angle-dependence of the magnetic fluctuations with another striking property of UCoGe, which is that its superconductivity is exceptionally sensitive to the direction of an applied magnetic field. When the magnetic field is perpendicular to the c axis the superconductivity is very robust, surviving to around 10 tesla; however, as the field direction is rotated towards the c axis, the critical field for destruction of superconductivity falls precipitously. An obvious interpretation of this behavior would be that the component of the applied field that is parallel to the c axis induces a large magnetic polarization, and the large internal field thus generated disrupts the paired electrons either through coupling to their spins or their orbital motion. This sort of physics is very well understood (indeed this is why ordinary superconductors don’t like magnetic fields) so it can be modeled quite accurately and, surprisingly, it doesn’t fit the measurements in UCoGe. Rather, Hattori et al. argue that their results are better explained if the magnetic field is disrupting not the pairs directly, but rather the underlying pairing mechanism. This, in particular, explains the striking parallel in the suppression of the magnetic fluctuations and the suppression of the superconductivity as the magnetic field is rotated towards the c axis. It is strong evidence that magnetic fluctuations are the ones doing the pairing.

A talk by P A Lee on SC and FM coexisting in oxide interface

http://videochannel.ust.hk/Watch.aspx?Section=Channels&Channel=2&SubType=All&View=Icon&Sort=Date&Page=1&Current=3&Mode=Play

A novel approach to cuprate SC ?

Here is a seemingly very interesting paper recently published in PRX. It contains an exotic treatment of the superconducting gap (not the pseudogap) in LSCO.

A generic theory of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental ‘‘second gap’’ in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering) processes, with a size unrelated to the
pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure dx2 y2 wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point
toward a more coherent understanding of high-temperature superconductivity.

Majorana fermions possibly in topological insulators

This is a wonderful short review of the work done in looking for those elusive fermions in condensed matter community. I'll almost utterly post it here.

The search for Majorana fermions is quickly becoming an obsession in the condensed-matter community. To understand the intense interest, I will begin with a practical definition: a Majorana fermion is a fermion that is its own antiparticle. While sophisticated particle physics experiments are testing for Majorana character in neutrinos propagating in three dimensions [1], solid state physicists are more interested in lower dimensional counterparts. The most interesting Majorana fermions that are predicted to appear in materials are zero-dimensional bound states confined to live on various types of topological defects [2]. In a paper published in Physical Review Letters, Pavan Hosur and collaborators from the University of California, Berkeley, predict that these bound states are found in the vortices of the superconductor Cu_xBi₂Se₃ [3] (Fig. 1). Once discovered, a set of zero-dimensional Majorana bound states (MBS) are predicted to exhibit exotic non-Abelian statistics when exchanged among each other. While of great fundamental interest, perhaps the biggest driving factor in the search is a well-regarded proposal for (topological) quantum computation, which uses this unique statistical property of the MBS to robustly process quantum information free from local sources of decoherence [4, 5].

....

Naively, this eliminates all fermions at play in conventional electronic systems from being Majorana. The key to getting around this obstacle is noting that one finds many different emergent fermionic vacua/ground states in electronic systems that are qualitatively different from the fundamental vacuum of spacetime. To illustrate this, consider a BCS superconductor ground state filled with a condensate of paired electrons. If we again scatter two electrons off each other, they can indeed bind into a Cooper pair and “annihilate” into the fermionic vacuum! However, if the vacuum is of s-wave character, the most common superconducting ground state, then the two electrons bound into the Cooper pair must have opposite spin and are thus not Majorana (the antiparticle of an electron with spin up, in this case, is one with spin down). The solution to this problem is manifest: we must find a way to get around the spin-quantum number. Currently, there are two primary mechanisms to do this: (i) the superconducting vacuum can have spin-triplet pairing, which pairs electrons with the same spin or (ii) the superconductivity can exist in the presence of spin-orbit coupling or some other mechanism which will remove the spin conservation. Solution (i) is the paradigm for the first proposals of the existence of MBS as quasiparticles of a fractional quantum Hall state which models a two-dimensional electron gas at filling ν=5/2 [6], and as vortex excitations in some theories of the unconventional superconducting state of Sr₂RuO₄ [7]. These proposals offer real material candidates for finding MBS, but experiments in both of these systems require utmost care in sample production and measurement precision. To date, MBS excitations have not been clearly distinguished in either of these systems. Recently, solution (ii), which was first implemented by Fu and Kane in topological insulator/superconductor heterostructures [8], has been garnering attention due to more inherent practicality. This has been followed up nicely with further predictions of MBS in low-dimensional spin-orbit-coupled heterostructures in proximity to s-wave superconductors [9].

The seminal proposal of Fu and Kane predicts that if the surface of a three-dimensional topological insulator is proximity-coupled to an s-wave superconductor, then vortex lines in the superconductor will trap MBS where the lines intersect the topological insulator surface [8]. This proposal requires two main ingredients: (i) a topological insulator and (ii) an s-wave superconductor that can effectively proximity-couple to the surface of the topological insulator. Despite all of the recent publicity about the discovery of three-dimensional topological insulators [10], finding a suitable topological insulator for these experiments is still a difficult task. The reason being that, as of yet, there are no topological insulator materials that are completely insulating in the bulk, despite intense experimental programs dedicated to this task. The most commonly studied topological insulators are variations of either Bi₂Se₃ or Bi₂Te₃ , in which it has been difficult to tune the bulk to a completely insulating state [11]. Thus, while many experiments have confirmed the robust nature and structure of the surface states, these materials, having bulk carriers, are not true topological insulators.

It is then natural to ask, What is a doped topological insulator good for? While one hopes that many of the topological phenomena of the true insulating state might be manifested in some form in a doped system, many questions still remain unanswered. However, Hosur et al. have made a striking prediction that MBS can still be realized in doped topological insulators under certain mild conditions [3]. A true insulating state is important in the Fu-Kane proposal because if the bulk contains low-energy states then the MBS can tunnel away from the surface and delocalize into the bulk, which effectively destroys the MBS. Hosur et al. circumvent this delocalization by requiring that the entire doped topological insulator become superconducting. They show that as long as the doping is not too large, vortices in superconducting topological insulators will bind MBS at the places where the vortex lines intersect the material surfaces. While this might seem like a big leap in complexity, experimental evidence already shows that, indeed, copper-doped Bi₂Se₃ is a superconductor below 3.8 K [12]. In this context, Hosur et al. make a strong prediction that vortex lines in superconducting Cu_xBi₂Se₃ can harbor MBS.

To understand the prediction, we begin with the Fu-Kane proximity effect scenario, as mentioned above, with a vortex line stretched between two surfaces. MBS are trapped where each end of the vortex line meets the topological insulator surface (see Fig. 1). If we tune the bulk chemical potential to lie in the conduction band, as opposed to the nominal insulating gap, then the MBS on each end of the vortex line could tunnel through the bulk and hybridize with the state on the opposite end. This is prevented in Hosur et al.’s work by inducing a superconducting gap in the entire bulk so that the MBS remain trapped. If the superconducting state were homogeneous, then the MBS would be trapped on the ends of the vortex line for any doping level. However, the superconducting order parameter varies rapidly near the vortex core, which is essentially a thin tube of normal metal (doped topological insulator) containing bound states with energies that lie below the nominal superconducting gap. It is easiest for the MBS to tunnel through the “mini-gap” region in the vortex core, and in fact, Hosur et al. go on to show that there is a critical chemical potential level where a vortex-core bound state becomes gapless and the MBS can easily tunnel through the vortex line to annihilate. Beyond this critical doping, the vortex line re-enters a gapped phase, but the MBS are absent. See Fig. 1 for an illustration of this process. The critical chemical potential can be calculated solely from low-energy information about the Fermi surface, and depends on the orientation of the vortex line with respect to the crystal structure. It is estimated that vortex lines oriented along the c axis of Cu_xBi₂Se₃ are just on the trivial side of the transition, while vortices perpendicular to the c axis should be well within the nontrivial regime and should trap MBS.

No concensus

A glance at how fierce the quarrels over the working mechanism of high Tc superconductors are !

No one is predicting a full understanding of high-temperature superconductivity any time soon — not least because such an account would have to make sense of the huge number of papers. “A rich enough theory should explain everything and not just cherry pick,” says David Pines, a physicist from the University of Illinois at Urbana-Champaign.
But it’s not always clear exactly what needs to be explained. Roughly 15 years ago, for example, researchers discovered that some high-temperature superconductors allow electron pairs to form above the transition temperature. In this ‘pseudogap’ regime, the material
spontaneously organizes itself into stripes: linear regions that act like rivers and carry electron pairs through the insulating landscape where electrons remain stuck in place. “It’s a precursor state to the superconducting state and is therefore fundamental to understanding this problem,” says Ali Yazdani, a physicist at Princeton University. Not so, says Pines, who thinks the pseudogap state “interferes with superconductivity but is not responsible for it”.
Much as physicists had to wait for highly developed quantum-mechanical tools to unlock the secret behind traditional superconductivity, researchers today may require future ideas to complete their task.
If nothing else, the field’s early quarrels have ensured that only the most determined researchers have stayed. Those remaining are perhaps humbled by their experiences. “I think our biggest problem has been human fallibility,” says Anderson. And perhaps these initial difficulties have helped to forge theories that can stand the test of time. “In the end, it’s your competitor that makes you strong,” says Shen

Nesting not so holy in pnictides

This [Phys. Rev. B 83, 220504 (2011)] might be call theories solely based on nesting into question !

Despite intense study, researchers have not yet uncovered the secrets behind the peculiar properties of iron-based (pnictide) superconductors. Many theories that try to explain the driving mechanism of superconductivity in these materials suggest it is tied to so-called nesting of the electron and hole Fermi surfaces. This geometric feature of the Fermi surface, where one portion of the surface maps to another if it is translated by a suitable reciprocal-lattice vector, is common to the structure of many families of pnictides. Nesting often implies the existence of collective electron behavior, so if it is present in the host materials of the pnictides, it would have significant implications for their properties.

In a Rapid Communication appearing in Physical Review B, Brendan Arnold at the University of Bristol, UK, and colleagues use the de Haas-van Alphen effect, where electrons and holes orbit the extrema of the Fermi surface in response to a magnetic field, to map out the electron and hole Fermi surface sheets of BaFe₂P₂, the parent material of an important family of pnictide materials. Besides providing highly detailed information about the geometry of the Fermi surfaces, they find, rather surprisingly, that the nesting present in the superconducting doped compounds BaFe₂(As_1-xP_x)₂ persists in BaFe₂P₂, which is not superconducting. This finding agrees with a growing list of experiments that conclude nesting does not play a dominant role in the development of superconductivity, at least in one family of pnictide compounds. – Alex Klironomos

Cohen in a lecture

He gave a lecture on his recent work, in which he has listed a handful of his work on graphene, on photovoltaics, nano structures and superconductors. It might be worthy to put the link here: http://videochannel.ust.hk/Watch.aspx?Section=Channels&Channel=2&SubType=All&View=Icon&Sort=Date&Page=3&Current=30&Mode=Play

Superconductivity at its centenery

In a previous entry, I have mentioned that, this year witnesses the 100th anniversary. Many scientific journals and magazines are piling special sections on this event. Here are what come from Science:

• Superconductivity's Smorgasbord of Insights: A Movable Feast

  • Adrian Cho

  Science 8 April 2011: 190-192.
  • Summary
  • Full Text
  • Full Text (PDF)
• News Search for Majorana Fermions Nearing Success at Last?

  • Robert F. Service

  Science 8 April 2011: 193-195.
  • Summary
  • Full Text
  • Full Text (PDF)
• The Challenge of Unconventional Superconductivity

  • Michael R. Norman

  Science 8 April 2011: 196-200.
  • Abstract
  • Full Text
  • Full Text (PDF)
• The Electron-Pairing Mechanism of Iron-Based Superconductors

  • Fa Wang and
  • Dung-Hai Lee

  Science 8 April 2011: 200-204.

A century after Onne's discovery

This year witnesses the centenary of superconductivity, which has spawned incredibly rich physics not limited to cryogenic field. Look at this review [http://www.nature.com/nphys/journal/v7/n4/full/nphys1981.html?WT.ec_id=NPHYS-201104]:

But continuing work on fundamental superconductivity is not the only legacy of the original discovery. Nor is the research concentrated on the high-temperature superconductors. On the contrary, organic superconductors, heavy fermions and ruthenates all continue to hold secrets. Over the past century, many ideas spawned by superconductivity have influenced or directly led to whole new fields of research. These include the study of helium-3, both for its cryogenic applications and multiple superfluid phases — Landau's Fermi liquid theory was originally proposed to explain the properties of helium-3. The study of non-Fermi liquids, with several examples of quantum criticality, is another active field.

In fact, these are all examples of strongly correlated electron systems, in which the whole is greater than the sum of its parts. Or, to quote Philip Anderson: “more is different”. Such materials exhibit all kinds of unexpected behaviour such as geometric frustration, glassy dynamics and metal–insulator transitions, to name but a few. Work in low-dimensional systems, including the study and manipulation of heterointerfaces within a superlattice structure, is also ongoing; and superconductivity even has an important role to play in the search for Majorana fermions in topological insulators.

Celebrating 100 years of superconductivity

This year marks the 100th anniversary since superconductivity was discovered in Leiden, The Netherlands, by Heike Kamerlingh Onnes and co-workers on 8 April 1911. Yielding no less than seven Nobel Prizes, the study of superconductors remains more active than ever in terms of forming a fundamental understanding of their underlying mechanism, and in seeking new and novel applications that already extend to digital electronics, sensors, medicine and metrology.

In recognition of this centennial year we are pleased to present a collection of superconductivity-themed review articles published in Reports on Progress in Physics over the last 10 years. Reflecting the wide-reaching impact of superconductors across many areas of physics, each article will be free to read until the end of 2011.

Tim Smith
Senior Publisher

Electrons take on diverse jobs in a compound

It was reported that two species of electrons in the same compound were found carrying superconductivity and anti-ferromagnetism, respectively. These two have different mass, one very light while the other quite heavy. This compound has quasi-3D stacked structure, nearly isomorphic to pnictide superconductors. I feel this is funny and many other rich phenomena might happen. "We found that the ZrCuSiAs-type crystal CeNi0:8Bi2 with a layered structure composed of alternate stacking of ½CeNixBið1Þ þ and Bið2Þ exhibits a superconductive transition at 4 K. The conductivities, magnetic susceptibilities, and heat capacities measurements indicate the presence of two types of carriers with notable different masses, i.e., a light electron responsible for superconductivity and a heavy electron interacting with the Ce 4f electron. This observation suggests that 6p electrons of Bi(2) forming the square net and electrons in CeNixBið1Þ layers primarily correspond to the light and heavy electrons, respectively."[PRL, 106:057002(2011)]

Preformed Cooper pairs become localied in the presence of disorder

Enough disorder leads to localized waves, a celebrated assertion due to Anderson. What will happen to superconducting Cooper pairs if disorder is added ? [http://www.nature.com/nphys/journal/v7/n3/full/nphys1892.html?WT.ec_id=NPHYS-201103]

The most profound effect of disorder on electronic systems is the localization of the electrons transforming an otherwise metallic system into an insulator. If the metal is also a superconductor then, at low temperatures, disorder can induce a pronounced transition from a superconducting into an insulating state. An outstanding question is whether the route to insulating behaviour proceeds through the direct localization of Cooper pairs or, alternatively, by a two-step process in which the Cooper pairing is first destroyed followed by the standard localization of single electrons. Here we address this question by studying the local superconducting gap of a highly disordered amorphous superconductor by means of scanning tunnelling spectroscopy. Our measurements reveal that, in the vicinity of the superconductor–insulator transition, the coherence peaks in the one-particle density of states disappear whereas the superconducting gap remains intact, indicating the presence of localized Cooper pairs. Our results provide the first direct evidence that the superconductor–insulator transition in some homogeneously disordered materials is driven by Cooper-pair localization.

Failed theories of SC

In one of my previous entries, I mentioned the failed theories that were briefly reviewed in a preprint. Here in Nature Physics [Nature Physics, 6: 715, 2010], another one based on this review came out. I especially like this story about Laudau:

Yet it turns out that Landau first proposed these ideas in the context of superconductivity, thinking not of magnetization but of electrical current. He expanded the free energy F around the state of zero current, j = 0, and argued that as the direction of the current shouldn't affect F, the odd terms should vanish. This gives an equation of the form F(j) = F(0) + aj² + bj⁴. Assuming b > 0 and that a passes through zero at a critical temperature T_c, he showed that there could be an abrupt transition from zero to non-zero current below T_c.

This early theory conflicted with observations — it erroneously predicted j ~ (T_c − T)^1/2 just below the critical temperature — and Landau went back to the drawing board. Yet here already were the seeds of the later Ginzburg–Landau theory of phase transitions. And Landau's introduction of the notion of an 'order parameter' as a convenient handle on order and how it changes has influenced physics ever since, even if it did appear in a failed theory.

This idea is crazy: when one expands free energy in current, one has in his mind that he is dealing with an equilibrium state. However, current usually exists in a non-equilibrium state. This gives a glimpse of the aberration of superconductivity !

Producing FFLO states

Nature, 467: 535–536:

Atomic gases cooled down to nanokelvin temperatures and confined in optical or magnetic traps have helped to realize and investigate fundamental many-body quantum phases of matter^{1, 2}. An investigation by Liao et al.³ on page 567 of this issue now shows how such ultracold systems are also moving to centre stage in the quest for an exotic form of superconductivity — the elusive FFLO superconducting state of matter that was proposed more than 40 years ago by Fulde and Ferrell⁴ and Larkin and Ovchinnikov⁵.

In condensed-matter physics, an arbitrarily small attraction between fermions (particles with half-integer spin, such as electrons) of identical but opposing spin and momentum can lead to the formation of bound pairs that have bosonic character (bosons being particles with whole-integer spin). Under specific conditions, such pairs can undergo the phenomenon of Bose–Einstein condensation (BEC), transforming the many-body system into a 'giant matter wave' with spectacular frictionless-flow properties — a superconductor or superfluid is born. This remarkable outcome of pairing, first proposed by Bardeen, Cooper and Schrieffer (BCS), is considered to be the conventional way in which superconductivity emerges in a wide range of materials. In the world of atomic physics, the same pairing mechanism has been studied thoroughly in three dimensions with equal two-component gas mixtures of fermionic neutral atoms^{1, 2}, each component comprising atoms with one of two spin states (up or down). But what happens to such a BCS superfluid state if the two fermionic spin states are not present in equal numbers in the system?

In a solid-state material, such a spin-imbalance condition can be created by applying a magnetic field to the system. In ultracold atomic gases, a simple initial difference in the number of spin-up and spin-down atoms will do the job. Intuitively, one might think that an increasing mismatch in the number of spin-up and spin-down particles would make it harder for the opposing spins to meet each other and pair up, thus hindering superconductivity. And this is indeed what happens in experiments. Put in more technical terms, the Fermi surfaces of the two system components will have different sizes, and this difference will hamper the formation of the pairs and the ensuing BCS superfluid state (the Fermi surface is the boundary in momentum space that separates unoccupied states from occupied ones).

Fulde and Ferrell⁴, as well as Larkin and Ovchinnikov⁵, proposed a clever solution that would still allow a superfluid state to exist under spin-imbalanced conditions. They suggested a paired state in which the pairs are not at rest but instead have a net momentum. This FFLO state can be viewed as a kind of microscale phase separation, containing alternating superfluid regions and normal, non-superfluid regions, in which the extra atoms of the spin species that are in excess squeeze in. Although searches for such an exotically paired FFLO state have been carried out exhaustively in condensed-matter systems, and more recently in ultracold atomic gases, unambiguous experimental evidence has remained elusive. In their study, Liao et al.³ take a major step towards creating an FFLO state using ultracold fermionic atoms.

Unconventional paring in inversion-symmetry lacking crystal

Electrons pair to carry supercurrent in a crystal. The paring symmetry can be s-wave, p-wave, d-wave and so on, depending on the underlying crystal structure. Those paring possibilities have definite spatial parities: s- and d-wave don't change under inversion while p changes sign and they don't mix in a centrosymmetric crystal. Moreover, they pair constrained by Pauli principle, which requires them to be a singlet for even-parity but triplet for odd-parity. However, for a non-centrosymmetric crystal, mixing is possible:

Unconventional pairs

Unconventional superconducting phase in the weakly correlated noncentrosymmetric Mo₃Al₂C compound: E. Bauer, G. Rogl, Xing-Qiu Chen, R. T. Khan, H. Michor, G. Hilscher, E. Royanian, K. Kumagai, D. Z. Li, Y. Y. Li, R. Podloucky, and P. Rogl ,Phys. Rev. B 82, 064511 (Published August 17, 2010)

Structure and physical properties of the noncentrosymmetric superconductor Mo₃Al₂C: A. B. Karki, Y. M. Xiong, I. Vekhter, D. Browne, P. W. Adams, D. P. Young, K. R. Thomas, Julia Y. Chan, H. Kim, and R. Prozorov: Phys. Rev. B 82, 064512 (Published August 17, 2010)

In superconductors, the appearance of dissipationless current is related to the formation of electron pairs with opposite spin and momentum. The symmetry of these pairs, which is constrained by the symmetries of the underlying crystal structure, defines important aspects of the superconducting state. So what happens to superconductivity when electron pairing occurs in a crystal structure that has no center of inversion?

This interesting question has been investigated in detail theoretically, and it was realized that in such cases the superconducting pairing is unconventional. In conventional (centrosymmetric) superconductors there can only be either spin-singlet or spin-triplet electron pairing, but in the absence of space-inversion symmetry the two can mix by the action of spin-orbit interaction (a relativistic effect), leading to unusual superconducting behavior.

This theoretical prediction has been tested experimentally in two independent articles that appear in Physical Review B. Ernst Bauer and collaborators from the Vienna University of Technology, Austria, with collaborators from China and Japan in one group, and Amar Karki and collaborators from Louisiana State University, US, with collaborators from Iowa State University, US, in the other, successfully grow and characterize Mo₃Al₂C. This material crystallizes in a noncentrosymmetric structure and undergoes a superconducting transition at T_c~9 K. Both groups observe signs of unconventional pairing, hinting at a strong connection between noncentrosymmetry and unconventional superconductivity. – Athanasios Chantis

More on the story of SC

Just came back from a short vacation. There is more on how the superconductivity was made by Onnes. An interesting story to read here !

Through the years leading to the discovery of BCS theory

Almost half a century passed between the discovery of superconductivity by Kammerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen, Cooper and Schrieffer. During the intervening years the brightest minds in theoretical physics tried and failed to develop a microscopic understanding of the effect. A summary of some of those unsuccessful attempts to understand superconductivity not only demonstrates the extraordinary achievement made by formulating the BCS theory, but also illustrates that mistakes are a natural and healthy part of the scientific discourse, and that inapplicable, even incorrect theories can turn out to be interesting and inspiring.

http://arxiv.org/ftp/arxiv/papers/1008/1008.0447.pdf

Spin-triplet pairs in the proximity of a supercondutor and a ferromagnet

Electrons in a supercondutor below Tc are described by a single wave function, which satifies Schrodinger's equation (dBG equation). The value of this function relates to the effective potential felt by the Copper pairs. The potential is negative in the superconductor but positive in a normal material. Thus, normal mateials make a energy barrier that prevents the peameability of electron pairs , and hence supercurrent to spill much, especially when the normal material is ferromagnetic. A ferromagnet favors spin-triplet, while Copper pairs are singlets. Nevertheless, there are some recent experiments observing a longer-range spilling of supercurrent, which was being put under the context of spin-triplet paring that may arise the perephery of an S-N interaface. Now, it came a paper aiming at this problem[1]. They found further evidences of such paring.

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

[1]Science 2 July 2010: Vol. 329. no. 5987, pp. 59 - 61; DOI: 10.1126/science.1189246

Type-2 superconductors used as tweezers

A just published work (PRA, 81, 063408, 2010) did calculations on how type-2 superconductors might be used to trap atoms or other tiny particles. In comparison with other choices, the scheme there-proposed has less loss of coherences. Their calculations proved type-2 SC as a very versatile device in that direction. What holds the key as tweezers is the formation of vortice, which allows the existence of local magnetic field. Such vortice appear in type-2 SC, but not in type-1.

A new superconductor: LaNiC

I knew nothing about this supercondutor until I had just a small poster about it. I found this poster on the following blog site:
http://blogs.kent.ac.uk/strongcorrelations/

This superconductor seems abberent in its symmetry, as perfectly summarized in that poster, which I'd like to share in my blog.