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.

Array of Graphene dots absorbs light perfectly

http://physics.aps.org/articles/v5/12

http://prl.aps.org/abstract/PRL/v108/i4/e047401

The results may be used in e.g. solar cells ! How to broaden the absorption spectrum?

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

Resistance becomes lower under pressure

The property of matter often changes drastically as external knobs such as temperature (goes lower) and pressure (gets raised) are tuned (http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.026403).

Wustite (FeO) is a prototype for the iron-bearing minerals found in the Earth. Though FeO is insulating at ambient conditions, in the late 1980s researchers observed it undergo a transition to a metallic state when compressed by shock waves. The nature of this transition has, however, been unclear.

In a paper in Physical Review Letters, Kenji Ohta of Osaka University, Japan, and colleagues report their combined theoretical and experimental attack on the problem. The research team measured high-temperature resistivity and structural x-ray diffraction patterns of FeO in a diamond anvil cell to simulate conditions in Earth’s mantle. At a temperature of 1900 kelvin and pressure of 70 gigapascals, Ohta et al. were able to watch as FeO in a rocksalt atomic structure became metallic without any structural changes.

To understand these findings, Ohta et al. performed density-functional calculations of electrical conductivity as a function of temperature and pressure. The results suggest that their observations are consistent with a new kind of insulator-metal transition involving fluctuations between a high-spin state to a low-spin state in the FeO. For geophysicists, this makes the picture of conductivity deep in the Earth richer: both insulating and metallic phases must be added to the phase diagram, with potential implications for thermal and electrical conductivity, and in turn models of the planetary magnetic field. –David Voss

Casimir force between two sheets of graphene

I had thought about this sometime ago but did not come up with the calculations on this problem, basically i did not get time to do that. Now i'm glad that some people have done this. See the following:
1. http://prb.aps.org/abstract/PRB/v80/i24/e245406
2. http://prb.aps.org/abstract/PRB/v82/i15/e155459
3. http://benasque.org/2011qfext/talks_contr/2021_Sernelius.pdf
4.http://arxiv.org/abs/1102.1757

Supersolid in history

Supersolid has been a fascinating conception since its inception. However, the experimental aspects are still not conclusive, as reviewed by Thouless in his private note:
http://www.phys.washington.edu/users/thouless/INItalk0808.pdf

More on the LAO/STO interface

This interface is really booming interests in similar structures ! See a summary by A.J.Millis of what might be expected in this system.

Electronic phase separation at the LaAlO(3)/SrTiO(3) interface
Authors: Ariando et. al.
Nature Communications 2 Article 188 (2011).
Coexistence of Superconductivity and Ferromagnetism in Two Dimensions
Authors: D. A. Dikin, M. Mehta, C. W. Bark, C. M. Folkman, C. B. Eom, and V. Chandrasekhar.
Phys. Rev. Lett. 107 056802 (2011).
Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3
interfaces
Authors: Lu Li, C. Richter, J. Mannhart and R. C. Ashoori
Nature Physics, doi 10.1038/nphys2080.
Direct Imaging of the coexistence of ferromagnetism and superconductivity at the
LaAlO3/SrTiO3 interface
Authors: J. A. Bert, B. Kalisky, C. Bell. M. Kim, Y. Hikita, H. Y. Hwang and K. Moler
Nature Physics, doi 10.1038/nphys2079

Interactions in graphene

The electrons in graphene are particularly nimble and their interactions are usually ignored together with the effects of some defects. However, recently more and more attention have been paid to them, e.g., about superconductivity and other new orders. Here is a brief summary by P. Guinea [http://www.condmatjournalclub.org/wp-content/uploads/2011/09/JCCM_SEPTEMBER_2011_01.pdf].

1. Broken symmetry states and divergent resistance in suspended
bilayer graphene
Benjamin E. Feldman, Jens Martin, Amir Yacoby,
arXiv:0909.2883, Nature Physics 5, 889 (2009)
2. Local Compressibility Measurements of Correlated States in
Suspended Bilayer Graphene
Jens Martin, Benjamin E. Feldman, R. Thomas Weitz, Monica T. Allen,
Amir Yacoby,
arXiv:1009.2069, Phys. Rev. Lett. 105, 256806 (2010)
3. Interaction-Driven Spectrum Reconstruction in Bilayer Graphene
A. S. Mayorov, D. C. Elias, M. Mucha-Kruczynski, R. V. Gorbachev, T. Tu-
dorovskiy, A. Zhukov, S. V. Morozov, M. I. Katsnelson, V. I. Fal'ko, A. K.
Geim, K. S. Novoselov,
arXiv:1108.1742, Science 333, 860 (2011)
4. Transport Spectroscopy of Symmetry-Broken Insulating States
in Bilayer Graphene
J. Velasco Jr., L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C.N.
Lau, C. Varma, R. Stillwell, D. Smirnov, Fan Zhang, J. Jung,
A.H. MacDonald
arXiv:1108.1609

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.

Two pieces of work on cuprates

Just to highlight them here, because they seemingly represent something defying the cliche.
1. Electron-spin excitation coupling in an electron-doped copper oxide superconductor [Nphy, 7:719(2011)]

High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations, which provides clues to the mechanism of high-Tc superconductivity. Here we use neutron scattering and scanning tunnelling spectroscopy (STS) to study the evolution of the bosonic excitations in electron-doped superconductor Pr0:88LaCe0:12CuO4􀀀 with different transition temperatures (Tc) obtained through the oxygen annealing process.We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the low-energy electron tunnelling modes detected by STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometre length scales, and the dominant electron–boson coupling at low energies originates from the electron-spin excitations.

2. Intense paramagnon excitations in a large family of high-temperature superconductors [Nphy 7:725(2011)]

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum on doping from the antiferromagnetic insulating to the superconducting state of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic X-ray scattering to show that a large family of superconductors, encompassing underdoped YBa2Cu4O8 and overdoped YBa2Cu3O7, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description of this surprisingly simple spectrum enables quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa2Cu3O7 reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to that of Eliashberg theories of conventional superconductors.

New clue toward paramagnons as the glue

Certainly lots of doubts are over the magnetic fluctuations as the paring source of carriers in cuprate superconductors. A central issues concerns if there is sufficient paramagnons in the Sc region, since experiments have so far detected only a limited volume of such stuff. Now this gets changed due to this work [Nature Phys. 7, 725–730 (2011). ] reviewed below [http://www.nature.com/nphys/journal/v7/n9/full/nphys2077.html?WT.ec_id=NPHYS-201109]:

However, the observed excitations were restricted to a narrow window in both energy and momentum and furthermore carried relatively little spectral weight, posing a challenge to theoretical ideas about magnetic fluctuations being the source of Cooper pairing in these superconductors. Some researchers have suggested that the experimental limitations inherent in neutron scattering were partially responsible for this state of affairs — and only now has a breakthrough occurred.

In Nature Physics, Le Tacon and colleagues² report the application to various copper oxides of an alternative technique to map magnetic excitations: resonant inelastic X-ray scattering (RIXS)³. Here, an electron is transferred, by a high-energy photon, from a deep core level into an unoccupied low-energy state; subsequently, an electron from a different low-energy state fills the core hole and emits a high-energy photon. Thus, a net excitation is generated in a low-energy band, the energy and momentum of which can be measured by examining the scattered photon.

Among the advantages of RIXS, compared with neutron scattering, is the large cross-section for the scattering of photons (which eliminates the need for large samples) and the possibility to probe essentially the entire Brillouin zone. There are disadvantages as well: in contrast to neutron scattering, the cross-section is not simply related to a dynamic susceptibility, which complicates the data analysis, and the energy resolution is at present limited to about 100 meV (it's far below 1 meV in state-of-the-art neutron-scattering experiments). Despite these limitations, the past decade has seen exciting progress in RIXS³ such that investigations of elementary spin excitations have now become feasible.

Le Tacon et al.² have investigated magnetic excitations using RIXS in a family of copper-oxide materials, covering a range of hole dopings from the undoped insulator to the slightly overdoped superconductor. In all doped materials, they identified damped spin excitations with high intensity over a large part of momentum space. These excitations, in both their overall dispersion and their intensity, seem to show surprisingly little variation with doping.

These findings are important for a number of reasons. First, together with similar recent experiments^{3, 4, 5}, they establish RIXS as a powerful tool for the investigation of complex correlated-electron materials. Second, they show that previous neutron-scattering studies have indeed missed a significant part of the spectral weight of spin fluctuations in copper oxides. This implies that theories of electron pairing based on the exchange of magnetic fluctuations can be considered on safer ground. In fact, Le Tacon et al. provide a sample calculation of a superconducting critical temperature (T_c), in which they use the measured spin-fluctuation spectrum and electronic bands as input and obtain a T_c value comparable to the experimental one.

Third, and perhaps most importantly, their data indicate that key features of the spin fluctuations in doped copper oxides are strikingly similar to that of their undoped counterparts (Fig. 1): at the elevated energies probed by RIXS, the only significant effect of doping is an energy broadening of the excitations, probably arising from damping due to electron-hole excitations. (One should note that the present energy resolution of RIXS is insufficient to resolve fine structures on scales below 100 meV; therefore the similarity of doped and undoped spectra refers to gross features, and the details may well differ.)

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.

BEC on an optical hexagonal lattice

The technology with lasers has incredibly enriched our understanding of a huge width of systems. The ability to prepare honeycomb lattice offers chances to study graphene-type physics and beyond. In this PRL paper, the authors addressed the issues of what would befall a Bose-Einstein condensate moving on a honeycomb lattice. They employed Gross-Pitaviskii equation, which is a mean-field theory for describing superfluids, to compute the band structure and found that, arbitrary interaction would drastically alter the structure around the Dirac points. Is it possible to observe similar stuff using graphene instead of an artificial lattice ? One needs to have superfluid to flow on graphene. The candidate is cooper pairs condensate, which may be created by placing a superconductor in contact with a layer of graphene.

The ability to prepare ultracold atoms in graphenelike hexagonal optical lattices is expanding the types of systems in which Dirac dynamics can be observed. In such cold-atom systems, one could, in principle, study the interplay between superfluidity and Dirac physics. In a paper appearing in Physical Review Letters, Zhu Chen at the Chinese Academy of Sciences and Biao Wu of Peking University use mean-field theory to calculate the Bloch bands of a Bose-Einstein condensate confined to a hexagonal optical lattice.

The Dirac point is a point in the Brillouin zone around which the energy-momentum relation is linear. Its existence in graphene is at the heart of this material’s unusual properties, in which electrons behave as massless particles. Chen and Wu’s study predicts, surprisingly, that in the analog cold-atom system, the topological structure of the Dirac point is drastically modified: intersecting tubelike bands appear around the original Dirac point, giving rise to a set of new Dirac points that form a closed curve. More importantly, this transformation should occur even with an arbitrarily small interaction between the atoms, upending the idea that such topological effects can only occur in the presence of a finite interaction between atoms.

The modified band structure prevents an adiabatic evolution of a state across the Dirac point, violating the usual quantum rule that a system remains in its instantaneous eigenstate if an external perturbation is sufficiently slow. This effect could be tested experimentally in a so-called triple-well structure, which is a combination of rectangular and triangular optical lattices. – Hari Dahal [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.107.065301]

Another simple and universal role in high Tc ?

These authors presented a very simple rule that seems validated by their analysis of experimental data [J. Phys.: Condens. Matter 23 (2011) 295701 (17pp)]. In this rule, the Tc of optimal compounds is essentially set by two length scales and the electron charge, i.e., Tc~e^2/l\times l'. What is striking is that, this rue was argued to cover a wide range of materials, including cuprates, pnictides and ruthenates. They proposed a paring mechanism via Compton scattering: e.g., the holes in the conducting layer is scattered by the electrons in the charge reservoir layer. Instead of forming excitons, superfluid forms. The following is a brief sojourn over this work [http://iopscience.iop.org/0953-8984/labtalk-article/46706]:

High-T_C superconductors have layered crystal structures, where T_C depends on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-T_C materials—cuprates, ruthenates, rutheno-cuprates, iron pnictides and organics—has revealed that the optimal transition temperature T_CO is given by the universal expression k_B^-1e²Λ / ℓζ. Here, ℓ is the spacing between interacting charges within the layers, ζ is the distance between interacting layers, Λ is a universal constant, equal to about twice the reduced electron Compton wavelength, k_B is Boltzmann's constant and e is the elementary charge. Non-optimum compounds in which sample degradation is evident typically exhibit T_C below T_CO. Figure 1 shows T_CO versus (ση/A)^1/2/ζ—a theoretical expression determining 1 / ℓζ, where σ is the charge fraction, η is the layer number count and A is the formulaic area. The diagonal black line represents the theoretical T_CO. Coloured data points falling within ± 1.4 K of the line constitute validation of the theory.

Figure 1. Experimental test of theory.

Figure 2. Proposed pairing interaction.

The elemental building block of high-T_C superconductors comprises two adjacent and spatially separated charge layers. The factor e² / ℓζ, determining T_CO arises from Coulomb forces between them. Remarkably an explicit dependence on phonons, plasmons, magnetism, spins, band structure, effective masses, Fermi-surface topologies and pairing-state symmetries in high-T_C materials is absent. The magnitude of Λ suggests a universal role of Compton scattering in high-T_C superconductivity, as illustrated in figure 2 that considers pairing of carriers (h) mediated by electronic excitation (e) via virtual photons (ν). Several other important predictions are given. A conducting charge sheet is non-superconducting without a second mediating charge layer next to it, and a charge structure representing a room-temperature superconductor yet to be discovered is presented.

Pseudogap does not twin with Superconducting gap: another evidence

I only have time to quickly graze over this interesting paper for the moment.

In underdoped cuprate superconductors, phase stiffness is low
and long-range superconducting order is destroyed readily by
thermally generated vortices (and anti-vortices), giving rise to
a broad temperature regime above the zero-resistive state in
which the superconducting phase is incoherent1–4. It has often
been suggested that these vortex-like excitations are related to
the normal-state pseudogap or some interaction between the
pseudogap state and the superconducting state5–10. However,
to elucidate the precise relationship between the pseudogap
and superconductivity, it is important to establish whether
this broad phase-fluctuation regime vanishes, along with the
pseudogap11, in the slightly overdoped region of the phase
diagram where the superfluid pair density and correlation
energy are both maximal12. Here we show, by tracking
the restoration of the normal-state magnetoresistance in
overdoped La2􀀀xSrxCuO4, that the phase-fluctuation regime
remains broad across the entire superconducting composition
range. The universal low phase stiffness is shown to be
correlated with a low superfluid density1, a characteristic of
both underdoped and overdoped cuprates12–14. The formation
of the pseudogap, by inference, is therefore both independent
of and distinct from superconductivity.

Smectic Coexisting with nematic in cuprate

In the pseudogap phase of cuprate superconductors, incredibly rich and exotic things have been observed, among which are the checkerboard pattern that breaks the C4v symmetry within an unit cell and the stripes that break an additional translational symmetry. These are called electronic nematic and smectic phases, respectively. According to this study, there should be an interesting interplay between the two on cuprates, due to topological defects. The authors formalize the coupling in a gauge invariant way.

Coupling to the smectic fields can then occur either through phase or amplitude fluctuations of the smectic. Here, we focus on the former, which means that couples to local shifts of the wave vectors and . Replacing the gradient in the x direction by a covariant-derivative-like coupling gives(4)and similarly for the gradient in the y direction, to yield a GL term coupling the nematic to smectic states. The vector represents by how much the wave vector, , is shifted for a given fluctuation. Hence, we propose a GL functional (for modulations along ) based on symmetry principles and and being small:(5)where … refers to terms we can neglect for the present purpose (SOM d). If we were to replace by where is the electromagnetic vector potential, Eq. 5 becomes the GL free energy of a superconductor; its minimization in the long-distance limit yields and thus quantization of its associated magnetic flux (22, 23). Analogously, minimization of Eq. 5 implies surrounding each topological defect (SOM e). Here, the vector is proportional to and lies along the line where = 0. The resulting key prediction is that will vanish along the line in the direction of that passes through the core of the topological defect, with becoming greater on one side and less on the other (Fig. 4B). Additional coupling to the smectic amplitude can shift the location of the topological defect away from the line of = 0 (SOM e).

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

Graphene age

In the past 20 years, we have gone through the copper age and iron age, both in superconductivity, weighing in strongly correlated systems. Some people might even have an impression that, few new physics can be found outside the U-regime. U is the Hubbard repulsion. But imagination leads us beyond that sight: we have found surprises and fostered new cherished babies in the usual band theory. We now are digging in the topological insulators and graphene, which are constantly offering exotic and practically important physics. The lesson here is: imagination is more important than knowledge ! (Einstein)

Classical Not Always Lose

It was shown that, classical physics does as efficiently as quantum physics in energy transfer in biological systems [http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.051911].

A prominent goal of quantum information and computing is to be able to exploit quantum entanglement in qualitatively new devices, such as massively parallel computers. Has biological evolution already harnessed entanglement for its own purposes? Recent studies have indeed suggested that electronic excitation transfer (EET) in photosynthesis benefits from quantum entanglement. Now, a paper appearing in Physical Review E is likely to stimulate further investigation and controversy on this question. Based on calculations, John Briggs and Alexander Eisfeld, of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, assert that under the conditions prevailing in photosynthesis (in particular, in the so-called Fenna-Matthews-Olson complex that lies at the heart of the process), energy transfer in a classical system is just as efficient as in its quantum counterpart.

To model the photosynthesis that occurs in plants, Briggs and Eisfeld study a collection of monomers, each possessing a single electronic state and coupled to its neighboring units by a dipolar interaction. The authors find that for dipolar interactions similar to those found in real molecular aggregates, the coherences in quantum transport (from the Schrödinger equation) are identical to those occurring in classical transport according to Newton’s equation. Although their analysis neglects the influence of the environment, the authors report that calculations including dephasing processes in the quantum and classical equations lead to the same conclusion. – Ron Dickman

Closed Line Of dislocations in graphene

Defects are valuables in materials science. They are purported to generate novel physics in unexpected ways. Low dimensional systems are more prone to defects. In graphene, structural defects can exist in zero or one D, called point defect or line defect, respectively. A line defect can have tremendous impact upon the transport properties of graphene. Very common is the dislocations. In this new work[PHYSICAL REVIEW B 83, 195425 (2011)], closed line of dislocations were observed. They look like flowers. They are topologically different from open dislocations. What kind of effects can be expected on electronic properties ? Let's see.

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves,
forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy or interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a “flower” pattern in scanning tunneling microscopy studies of epitaxial graphene grown on SiC(0001).We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations.