Type-IV Superconductivity: Can Superconductivity be more Exotic than Unconventional?

Abrikosov vortex phase in s-wave layered superconductor in a parallel magnetic field is shown to be absolutely unstable with respect to the appearance of a triplet component which breaks inversion (parity) and spin-rotational symmetries of Cooper pairs. Symmetry breaking paramagnetic effects are demonstrated to be of the order of unity if the orbital upper critical field, H_c2 (0), is of the order of Clogston paramagnetic limiting field, H_p. We suggest a generic phase diagram of such type-IV superconductor, which is singlet one at H=0 and in the Meissner phase and characterized by mixed singlet-triplet order parameter with broken symmetries of Copper pairs in vortex phase. A possibility to observe type-IV superconductivity in clean organic, high-Tc, and MgB₂ superconductors is discussed.     

Attractive Vortex Interaction and the Intermediate-Mixed State of?Superconductors

The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail, and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and other materials 40 years ago; last year it was possibly also observed in MgB₂, where it was called ??a totally new state?? and ascribed to the existence of two superconducting electron bands, one of type-I and one of type-II. The complicated electronic structure of MgB₂ and its consequences for superconductivity and vortices are discussed. It is shown that for the real superconductor MgB₂ which possesses a single transition temperature, the assumption of two independent order parameters with separate penetration depths and separate coherence lengths is unphysical.     

Superconductivity in Fe and As Based Compounds: A Bridge Between MgB<Subscript>2</Subscript> and Cuprates

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB₂, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB₂ and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps.     

Vortex Molecules in Thin Films of Layered Superconductors

Both the equilibrium and transport properties of the vortex matter are essentially affected by the behavior of the intervortex interaction potential. In isotropic bulk superconductors this potential is well known to be repulsive and is screened at intervortex distances R greater than the London penetration depth λ. As a result, in perfect crystals quantized Abrikosov vortices form a triangular lattice. In thin films of anisotropic superconductors this standard interaction potential behavior appears to be strongly modified because of the interplay between the long-ranged repulsion predicted in the pioneering work by J. Pearl and the attraction caused by the tilt of the vortex lines with respect to the anisotropy axes. This interplay results in a new type of vortex arrangement formed by finite-size vortex chains, i.e., vortex molecules. Tilted vortices with such unusual interaction potential form clusters with the size depending on the field tilting angle and film thickness or/and can arrange into multiquanta flux lattice. The magnetic flux through the unit cells of the corresponding flux line lattices equals to an integer number N of flux quanta. Thus, the increase in the field tilting (or varying temperature) should be accompanied by the series of the phase transitions between the vortex lattices with different N. A similar scenario should be realized in strongly anisotropic BSCCO high-T _c superconductors where in tilted field a crossing lattice of Abrikosov vortices (the stacks of pancakes in this case) and Josephson vortices appears. This crossing leads to the zigzag deformation of the pancakes stacks which is responsible for the attraction interaction competing with the long-ranged Pearl’s repulsion.     

Proximity Effect of Magnesium Diboride on Single-Walled Carbon Nanotube: an Ab Initio Study

Magnesium diboride (MgB₂) superconductor with excellent physical properties continues to attract the attention of researchers since its discovery. It derives its versatility from the absence of weak links, large coherence length, and small anisotropy. On the other hand, reports of superconductivity in small-diameter single-walled carbon nanotubes (SWCNTs) suspended between superconducting contacts and proximity induced supercurrents in Ta/SWNTs/Au junctions have also aroused great interest in the scientific community. Proximity induced superconductivity in SWCNTs has opened up new frontiers of research which will lead to many novel discoveries. This paper reports ab initio investigations on the proximity effect of MgB₂ on the electronic structure of a SWCNT. Condensation of electronic states is observed in the electronic band structure of the pristine SWCNT when MgB₂ is held in proximity. An additional band gap is generated below the lowest energy state of the valence band of the pristine CNT which we suggest, is due to Cooper pair formation. This leads to the prediction that SWCNTs will show superconducting properties in proximity of MgB₂. We envision MgB₂-coated SWCNTs as a novel nanomaterial that has a combination of proximity induced superconductivity and inherently unique mechanical and optical properties of SWCNTs.     

Possible High-Temperature Superconductivity in Multilayer Graphane: Can the Cuprates be Beaten?

We analyze a possible superconductivity in the hole-doped system of layered hydrogenized graphene by taking into account thermal fluctuations of the order parameter. In particular, we demonstrate that in the one-layer case the values of the high mean-field (MF) critical temperature $T_{\mathrm{\mathrm{c}}}^{\mathrm{MF}}\sim 80$ ?C90 K, predicted recently by Savini et al (Phys. Rev. Lett. 105:037002, 2010), do not alter significantly due to the fluctuations, and the Berezinskii-Kosterlitz-Thouless (BKT) critical temperature of the vortex superconductivity is almost the same as the MF temperature at doping 0.01?C0.1. We show that in the case of multilayer system, when the coupling between the layers stabilizes the superconducting phase in the form of fluxon superconductivity, the critical temperature T _c can increase dramatically to the values ??150 K, higher than the corresponding values in cuprates under ambient pressure.     

The surprising superconductor

It was last year, around this time, that physicists were stunned by the announcement that magnesium diboride, MgB₂, a material known since the 1950s, superconducts¹ at a critical temperature, T_c, of 39 K. Since this surprising discovery we have witnessed an explosion of research on MgB₂ and other related compounds to answer the following questions: is MgB₂ unique or are there other similar compounds with higher T_c; what is the mechanism of superconductivity; and what are the potential technical applications of this discovery? This brief report of the progress made in the first year of the MgB₂ era gives an insight into the answers to these questions.The serendipitous discovery by Akimitsu’s group¹ of the superconductivity of MgB₂ at T_c=39 K, almost twice the temperature of other simple intermetallic compounds, has sparked a race to uncover its basic properties and to find other related diborides with even higher T_cs. After the first announcement, the number of preprints appearing on the Los Alamos preprint server (Fig. 1

1. Download : Download high-res image (15KB)
2. Download : Download full-size image

Fig. 1. The number of studies about MgB₂ appearing in every two weeks in the xxx.lanl.gov e-print archives.

) grew almost exponentially, reaching a maximum of about 60 studies in March (two papers a day), then decreasing linearly down to a paper every other day in August, and staying steady at about this rate until now. During the first year of the MgB₂ era, more than 300 studies were published, exploring both fundamental and practical issues, such as the mechanism of the superconductivity; synthesis of MgB₂ in the form of powder, thin films, wires, and tapes; the effect on T_c of substitution with various elements and on critical current and fields.     

Vortex Molecules in Bose-Einstein Condensates

Stable vortex dimers are known to exist in coherently coupled two component Bose-Einstein condensates (BECs). We construct stable vortex trimers in three component BECs and find that the shape can be controlled by changing the internal coherent (Rabi) couplings. Stable vortex N-omers are also constructed in coherently coupled N-component BECs. We classify all possible N-omers in terms of the mathematical graph theory. Next, we study effects of the Rabi coupling in vortex lattices in two-component BECs. We find how the vortex lattices without the Rabi coupling known before are connected to the Abrikosov lattice of integer vortices with increasing the Rabi coupling. In this process, vortex dimers change their partners in various ways at large couplings. We then find that the Abrikosov lattices are robust in three-component BECs.     

Emergence of the Vortex State in Confined Ferroelectric Heterostructures

The manipulation of charge and lattice degrees of freedom in atomically precise, low‐dimensional ferroelectric superlattices can lead to exotic polar structures, such as a vortex state. The role of interfaces in the evolution of the vortex state in these superlattices (and the associated electrostatic and elastic boundary conditions they produce) has remained unclear. Here, the toroidal state, arranged in arrays of alternating clockwise/counterclockwise polar vortices, in a confined SrTiO₃/PbTiO₃/SrTiO₃ trilayer is investigated. By utilizing a combination of transmission electron microscopy, synchrotron‐based X‐ray diffraction, and phase‐field modeling, the phase transition as a function of layer thickness (number of unit cells) demonstrates how the vortex state emerges from the ferroelectric state by varying the thickness of the confined PbTiO₃ layer. Intriguingly, the vortex state arises at head‐to‐head domain boundaries in ferroelectric a₁/a₂ twin structures. In turn, by varying the total number of PbTiO₃ layers (moving from trilayer to superlattices), it is possible to manipulate the long‐range interactions among multiple confined PbTiO₃ layers to stabilize the vortex state. This work provides a new understanding of how the different energies work together to produce this exciting new state of matter and can contribute to the design of novel states and potential memory applications.     

Superconducting single crystalline MgB2 nanotubes

Large-scale single crystalline MgB₂ tubelike nanostructures were successfully prepared by thermal evaporation of MgB₂ particles precursors without involvement of template or patterned catalyst. The inner diameter, outer diameter and length of the as-fabricated single MgB₂ nanotube (NT) are respectively about 30 nm, 90 nm and several tens of microns. Existence of superconductivity within the products is confirmed by AC and DC magnetic susceptibility at low temperatures. The work represents the achievement to produce the bulk superconductivity with the hollow-structured morphology. Synthesis of MgB₂ NTs with bulk superconductivity may open up new possibilities for the fundamental understanding of the effect of dimensionality on superconductivity.     

Striped Organization of Hole Excitations and Oxygen Interstitials in Cuprates as a Route to Room-Temperature Superconductivity

There is increasing evidence that the adequate approach to the puzzle of high-temperature superconductivity in cuprates has to bear in mind the local lowering of crystal symmetry. According to the author??s model, the bound states of dopant ions and hole orbitals in lightly doped cuprates can be treated as pseudoatoms. Their alignment leads to formation of extended quantum protectorates??bosonic stripes (BS) characterized by a set of discrete widths. The simulations of electronic properties performed for La₂CuO_4+?? , YBa₂Cu₃O_6+?? , and HgBa₂Ca₂Cu₃O_8+?? in the framework of this scenario are consistent with experimental data. It is argued that the concept of BS hierarchy may help not only to clarify the heart of unconventional physics of cuprates but also to answer the crucial questions about perspectives to raise the superconducting critical temperature above 300?K.     

Vortex Motion and Quasiparticle Resistivity in MgB<Subscript>2</Subscript> at Microwave Frequencies

The electrical transport in superconductors at nonzero frequencies is affected by the normal and superfiuid fractions, as well as moving vortices, resulting in intricate expressions for the complex resistivity. In MgB₂, additional complications arise from the two-band nature of this material. We present an accurate study of microwave resistivity data measured in MgB₂ thin films by means of the Corbino disk broadband technique between 2 and 20 GHz. We show that a two-fluid model applies well in a relatively large region of the H–T phase diagram. Excellent agreement is found between the derived superfluid parameters (superfluid density, upper critical field) and theoretical predictions. In the same H–T region we isolate and discuss the vortex motion complex resistivity. To this end, we make use of the expressions given by the model by Coffey and Clem (CC). We show that the frequency dependence of the complex vortex resistivity recovers the CC model. However, the temperature and field dependence of the obtained parameters are at odds with the assumptions of the model. We discuss possible explanations of these oddities by considering collective pinning of vortices.     

Effect of Rare-Earth Oxides Doping on the Superconductivity and Flux Pinning of MgB<Subscript>2</Subscript> Superconductor

A series of rare-earth-oxide-doped MgB₂ bulks are prepared by in situ solid-state reaction with Pr₆O₁₁, CeO₂, Lu₂O₃, and Ho₂O₃ as the dopants. The superconducting properties are investigated and compared with the nano-Fe₃O₄ doped MgB₂. It is found that different from doping ferromagnetic nano-Fe₃O₄ which drastically suppresses superconductivity of MgB₂, doping the rare-earth-oxides has little effect on superconductivity of MgB₂ although most rare-earth elements have strong magnetic moment. In addition, some boride impurities formed during the reaction between rare-earth oxides and boron can work as effective pinning centers and significantly improve J _c and H _irr of MgB₂ when these fine nanoboride precipitates (<20 nm) are embedded into the MgB₂ intragrains.     

Plasma arc Synthesis of Nano-Boron Towards the Synthesis of MgB2 Superconductors

Since the discovery of superconductivity in the tempting binary intermetallic compound MgB₂, the solid-state synthesis technique is highly dominated by the usage of amorphous boron as one of the precursor powders. The formation of MgB₂ phase proceeds through the diffusion of Mg into B powder mainly driven by the low melting point of Mg as compared to B. Once the nucleation is achieved, the progress of polycrystalline MgB₂ phase occurs due to the out diffusion of boron through the MgB₂ layer and by the inward diffusion of Mg. This growth is impeded due to the presence of certain oxide phases or formation of Mg deficient phases. It is speculated that the probability for the inclusion of Mg(B)–O phases is higher for crystalline boron precursor as compared to the amorphous B. Thus, the use of nanosized amorphous boron may lead to larger nucleation centers, smaller grain size and consequently higher packing density in the polycrystalline MgB₂, which will in turn provide optimum superconducting properties. Hence an attempt to synthesize amorphous nano-boron powders is presented. Plasma arc discharge technique was successfully employed to produce nano-boron powder. The XPS analysis was carried out to inveterate the formation of boron. The as-synthesized powder had a uniform average particle size distribution of around 20 nm as confirmed by TEM measurements. The selected area electron diffraction pattern composed of diffused ring clearly depicts the amorphous nature of boron powder.     

The T c Amplification by “Shape Resonance” in Doped MgB2

We have applied two channels (σ and π) superconductivity model to the Al_1?x Mg _x B₂. Using the experimental data, we have calculated the strength of the interchannel pairing due to quantum interference effects, probed by the interband coupling parameter, and the two gaps as a function of the x. While in MgB₂ the quantum interference effects gives an amplification of T _c by factor 1.5 in comparison with the dominant intra σ band single channel pairing, in AlMgB₄ the amplification is about 100, in comparison with the dominant intra π band single channel pairing.     

Superconductivity and Spin Fluctuations

The organizers of the Memorial Session for Herman Rietschel asked that I review some of the history of the interplay of superconductivity and spin fluctuations. Initially, Berk and Schrieffer showed how paramagnon spin fluctuations could suppress superconductivity in nearly-ferromagnetic materials. Following this, Rietschel and various co-workers wrote a number of papers in which they investigated the role of spin fluctuations in reducing the T _c of various electron-phonon superconductors. Paramagnon spin fluctuations are also believed to provide the p-wave pairing mechanism responsible for the superfluid phases of ³ He. More recently, antiferromagnetic spin fluctuations have been proposed as the mechanism for d-wave pairing in the heavy-fermion superconductors and in some organic materials as well as possibly the high-T _c cuprates. Here I will review some of this early history and discuss some of the things we have learned more recently from numerical simulations.     

Anomalous raman scattering in the normal state of uederdoped crystals of YBA2Cu3O7-x

The integrated intensity of the electronic background between 200 and 700 cm^-1 observed in the Raman spectra of the YBa₂Cu₃O_7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO₂ nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity occur at temperatures greater thanT _c . The one at lower temperature is proportional toT _c and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements in other HT_c material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at T_D>- T_c (where T_D decreases as T_c increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity in cuprates.     

A Unified and Asymptotically Exact Theory of Superconductivity from Weak to Strong Coupling in an Electron–Phonon Model

Since the discovery of MgB₂, fundamental studies of the electronphonon mechanism have received intense interest. In this paper, starting from an electron–phonon model Hamiltonian, the third formalism of quantum statistics, and a diagonalization theorem, a unified and asymptotically exact theory of superconductivity is developed. The results are compared with those of McMillan, Allen and Dynes and experimental data. It is shown that the new results are in very good agreement with the experimental data. The unified theory has no extra assumptions and also supplies an exactly soluble example of the third formalism of quantum statistics. The T_c of MgB₂ is at a unified T_c line, the reason for the vacuum fluctuation mechanism are also discussed.     

Interplay Between Vorticity and Chirality Inside the Vortex Core in Chiral p-Wave Superconductors

The vortex core in chiral p-wave superconductors exhibits various properties owing to the interplay between the vorticity and chirality inside the vortex core. In the chiral p-wave superconductors, the site-selective nuclear spin-lattice relaxation rate T ^–1 ₁ is theoretically studied inside the vortex core within the framework of the quasiclassical theory of superconductivity. T ^–1 ₁ at the vortex center depends on the sense of the chirality relative to the sense of the magnetic field. The effect of a tilt of the magnetic field upon T ^–1 ₁ is investigated. The effect of the anisotropy in the superconducting gap and the Fermi surface is then investigated. The result is expected to be experimentally observed as a sign of the chiral pairing state in a superconducting material Sr₂RuO₄.