|Current Workshop | Overview||Back | Home | Search ||
100th Anniversary of Superconductivity: Hot Topics and Future Directions
Phase diagram of Fe based superconductors
Institut für Festkörperforschung, IFW Dresden; Institut für Festkörperphysik, TU Dresden
Focusing on REO1-xFxAsFe (RE = La, Sm, Ce) we have studied the phase diagram as well as magnetic and electronic properties of iron pnictide superconducting using a broad spectrum of experimental techniques, such as NMR µSR, ARPES, magnetometry, x-ray diffraction, and transport measurements. Details of the phase diagram , the magnetic ordering [2, 5], properties of the superconducting state [1, 3, 5] and the normal state electronic properties [1, 4, 6, 7] in the superconducting regions of the phase diagram are derived from these experimental studies. In the paramagnetic normal state, NMR on three different nuclei in LaO1-xFxAsFe shows that the local electronic susceptibility rises with increasing temperature. In addition, the relaxation rate as determined from NMR studies points to the presence of antiferromagnetic fluctuations, which are most pronounced in the underdoped systems close to the magnetically ordered phase. Moreover, measurements of the electrical field gradient by NQR yield clear-cut evidence for nanoscale order of charges and/or orbitals  which are reminiscent to the famous stripe order found in cuprates and other strongly correlated systems.
In addition I will report evidence for triplet superconductivity that we find in high quality single crystals of LiFeAs [9,10], which is a stoichiometric pnictide superconductor with Tc ~ 18 K. This evidence is obtained from the observation that the Knight shift, measured by 75As nuclear magnetic resonance, is unchanged across Tc for fields parallel to the Fe-As plane . This evidence is further supported by comparing our quasiparticle interference data which model calculations assuming several symmetries of the superconducting order parameter.
 H.-J. Grafe et al., Phys. Rev. Lett. 101, 047003 (2008), New J. Phys. 11, 35002 (2009)
 H.-H. Klauss et al., Phys. Rev. Lett. 101, 077005 (2008)
 H. Luetkens et al., Phys- Rev. Lett. 101, 097009 (2008)
 R. Klingeler et al., Phys. Rev. B , 81, 024506 (2010)
 H. Luetkens et al., Nature Materials 8 305, (2009)
 C. Hess et al., Europhys. Lett. 87, 17005(2009)
 V. Zabolotnyy et al., Nature 457, 569 (2009)
 G. Lang et al., Phys. Rev. Lett. 104, 097001 (2010)
 S.V. Borisenko et al., Phys. Rev. Lett. 105, 067002 (2010)
 I. Morozov et al., Cryst. Growth Des. 10(10), 4428 (2010)
 S.-H. Baek et al., preprint
 T. Hänke et al., preprint
Exploring broken electronic symmetries within the CuO2 unit cell
J.C. Séamus Davis
Direct visualization of the electronic structure within each crystalline unit cell is a new frontier in condensed matter physics (M. J. Lawler et al, Nature 466, 347 (2010); http://dx.doi.org/10.1038/nature09169). In this talk, I will describe studies of the intra-unit-cell broken electronic symmetry of the cuprate pseudogap phase, of the relationship between these intra-unit-cell phenomena and broken translational symmetry (A. Mesaros et al (2011), and of the connection between these phenomena and the high temperature superconductivity.
Superconductivity in ferromagnets URhGe and UCoGe
E. Yelland1,2, D. Sokolov1, W. Wang1, J. Barraclough2, K. Kamenev1, P. Bourges3, C. Stock4 and A. Huxley1,2
1SUPA, School of Physics and Astronomy, University of Edinburgh, King’s Buildings, Edinburgh EH9 3JZ,
2SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS,
3LLB-Orph´ee, CEA-Saclay, 91191 Gif-sur-Yvette, France
4NIST, 100 Bureau Drive, M/S 6100, Gaithersburg, MD 20899-8110 USA
Theoretically, longitudinal magnetic fluctuations can mediate odd-parity pairing of equal-spin fermionic electronic excitations giving superconductivity . Since this superconductivity can survive in the strong exchange field present in a ferromagnet this is a natural starting point from which to explain superconductivity in three ferromagnets UGe2 (under pressure)  , URhGe   and UCoGe . Experimentally, however, it is not established that longitudinal fluctuations drive pairing in these materials or to what extent special circumstances such as fermiology are important.
The presentation will focus on two new results:
Firstly, we report on the magnetic excitation spectrum in the ferromagnetic superconductor UCoGe measured with inelastic neutron scattering. The excitations are well described by overdamped modes with strong damping of the uniform magnetisation. Although this damping is unusual it is similar to that observed in the FM1 state of UGe2 . Our measurements for UCoGe reveal that the magnetic correlation length is anisotropic in contrast with findings for UGe2 at room pressure. This could help promote superconductivity in UCoGe and constrain the symmetry of the superconducting order-parameter.
Secondly, we report measurements of quantum oscillations for URhGe crossing the moment rotation transition at which superconductivity is induced. Our results reveal a significant change crossing this transition. This could be important for the formation of superconductivity and suggests that the actual mechanism giving superconductivity may contain ingredients that are not considered in the above theory.
 D. Fay and J. Appel, Phys. Rev. B 22 3173 (1980).
 S. Saxena, et al. Nature, 406 587 (2000).
 A. D. Huxley et al., Phys. Rev. B 63 (2001) 144519
 D. Aoki et al Nature 413 (2001) 613
 F. L´evy et al. Science 309 (2005) 1343
 N.T. Huy et al. Phys Rev Lett 99 (2007) 067006
 A.D. Huxley et al Phys Rev Lett 91 207201
Since the first demonstration of electric field modulation of superconductivity half a century ago , the electric field induced superconductivity has been one of the great challenges of solid state sciences and device physics. The basic concept is to tune Tc of superconductors through the carrier density control in the metal-oxide-semiconductor field-effect transistor (MOS-FET) devices. Interest in the electric field induced superconductivity comes from its cleanliness of field effect doping as well as from its potential as a novel method for searching for new superconductors. In the past ten or fifteen years, there have been great progresses in the field effect control of superconductivity, among which two notable streams are the field effects on strongly correlated systems  and Anderson insulators . The common characteristic of these two is that the samples are designed to be already superconducting or to be very close to the insulator-superconductor boundary, because the effect of electric field was not sufficient for converting real insulators to superconductors.
In this talk, we discuss a new opportunity of electric field induced superconductivity using electric double layer transistors (EDLTs). At the electrochemical interface, called electric double layer, extremely large electric fields are generated so that the one is able to accumulate enough amount of carriers for inducing superconductivity in insulators.
With the EDLT devices, we have demonstrated electric field induced insulator metal transition in ZnO , and superconductivity in SrTiO3  and ZrNCl . By utilizing ionic liquids  in the ELDTs, the accumulated sheet carrier density has now reached 1 x 1015 cm-2, which is large enough to induce superconductivity in many doped superconductors, and provides a variety of opportunities to manipulate the electronic states of interfaces by gate voltages.
The next thing to be tried is to induce superconductivity in materilas, which are not known to superconduct in bulk phases. KTaO3 has been known to show very similar behavior to SrTiO3, among which the quantum paraelectric nature is highly advantageous for having high mobility. However, no superconductivity has been reposted on KTaO3, probably because of the limitation of carrier doping. We demonstate that the EDLT device can dope carriers, the density of which can reach one order of magnitude higher than that of the bulk. The presnet result indicates that EDLT has a potential to realize a new electronic state which is not accessible by conventional chemistry.
This work has been carried out in collaboration with H. Shimotani, H. T. Yuan, J. T. Ye, Y. Kasahara, M. Kawasaki, A. Tsukazaki, T. Fulkumura (University of Tokyo), K. Ueno, T. Nojima, S. Nakamura (Tohoku University), and T. Hatano, M Nakano, S. Ono (RIKEN).
 C. H. Ahn, J. –M. Triscone, and J. Mannhart, Nature 424, 1015 (2003).
 K. L. Parendo et al., Phys. Rev. Lett. 94, 197004 (2005)
 H. Shimotani et al., Appl. Phys. Lett. 91, 082106 (2007).
 K. Ueno et al., Nat. Mater. 7, 855 (2008).
 J. T. Ye et al., Nat. Mater. 9, 125 (2010).
 H. T. Yuan et al., Adv. Funct. Mater. 19, 1046 (2009).
Measurement of the Hybridization gap in the Kondo lattice URu2Si2
Laura H. Greene
Department of Physics and the Frederick Seitz Material Research Laboratory, University of Illinois at Urbana-Champaign
The hybridization gap in the heavy fermion compound URu2Si2 is detected by point contact spectroscopic measurements. The distinct asymmetric double-peak structure, reproducibly observed in the conductance across ballistic junctions, constitutes the first direct evidence for a Fano resonance in a Kondo lattice, confirming recent theoretical predictions . The hybridization gap opens up well above the hidden order transition temperature, indicating that it is not an order parameter for this phase.
 M. Maltseva, et al., Phys. Rev. Lett. 103, 206402 (2009)
In collaboration with W. K. Park, P. H. Tobash, F. Ronning, E. D. Bauer, J. L. Sarrao, and J. D. Thompson
The work at UIUC is supported by the U.S. DOE under Award Nos. DE-FG02-07ER46453 and DE-AC02-98CH10886, and the work at LANL is carried out under the auspices of the U.S. DOE, Office of Science
Spin fluctuations-mediated pairing and organic superconductivity
Magnetism is very prominent in the generic phase diagram of quasi-one dimensional TMTSF2X organic superconductors discovered more than 30 years ago. It manifests itself by the existence of spin-Peirls, Néel and spin density wave states before superconductivity can be stabilized under high pressure.
Low frequency antiferromagnetic fluctuations are quite influential in the metallic state above the onset of superconductivity (enhancement of NMR relaxation, linear-in temperature contribution to the inelastic scattering rate, contribution of fluctuating SDW’s to the conduction). The close relation established under pressure between the linear scattering and superconductivity is a strong support for the existence of an antiferromagnetism-mediated pairing leading to a spin-singlet nodal superconductivity in agreement with recent experimental studies performed on pure and doped organic superconductors.
 Interacting elecrons in quasi-one-dimensional organic superconductors, C. Bourbonnais and D. Jérome, in The Physics of Organic Superconductors and Conductors, p 357, A.G. Lebed editor, Springer Series in Materials Science, Springer, Heidelberg, 2008.
 Correlation between linear resistivity and Tc in the Bechgaard salts and the pnictide superconductor Ba(Fe (1-x) Cox)2As2, N. Doiron-Leyraud et-al, Phys. Rev. B, 80, 214531, 2009 and Eur, Phys, Jour .B, 78,23, 2010. DOI: 10.1140/ep jb/e2010-10571-4
 Fluctuating spin density waves conduction in TMTSF2X organic superconductors, P. Auban-Senzier, C. R. Pasquier and D. Jérome, Eur. Phys. Lett, to be published.
 Thermodynamic obeservation of the nodal prder parameter of the quasi-one dimensioanl superconductor TMTSF2ClO4, S. Yonezawa, Y. Maeno, K. Bechgaard and D. Jérome, to be published.
 Impurity controlled Superconductivity/Spin Density Wave interplay in the organic superconductor : (TMTSF)2ClO4 , N.Joo,P.Auban-Senzier, C.Pasquier, D.Jérome and K.Bechgaard, Eur.Phys.Lett, 72,645,2005.
Presentation based on collaborations with N. Doiron-Leyraud, L. Taillefer, C. Bourbonnais,
P. Auban-Senzier, C. Pasquier, S. Brown, K. Bechgaard, S. Yonezawa and Y. Maeno
Mott transition and superconductivity in quasi-triangular-lattice organics
Strong electron correlation is recognized to mediate superconducting electron pairing. The layered organic systems k-(ET)2X with a variety of anions, X, provide playgrounds for Mott physics in two dimensions. Because the conducting layer consists of anisotropic triangular lattice with the anisotropy varied by X, spin frustration as well as Coulomb repulsion plays an important role in the nature of ground state. Depending on the anisotropy of the triangular lattice, an antiferromagnic state (for X=Cu[N(CN)2]Cl) or a spin liquid state (for X=Cu2(CN)3) appears in the Mott insulating phases, and both give way to superconducting state when pressurized or doped. In this workshop, I review the nature of the two Mott insulators, the 1st-order insulator-to-superconductor transition, pairing symmetry and pseudo-gapped behavior, shedding light on the role of electron correlation and spin frustration in the emergence of superconductivity.
This presentation is based on the work in collaboration with K. Miyagawa, Y. Shimizu, F. Kagawa, Y. Kurosaki, T. Furukawa, H. Hashiba, H. Oike, H. Kasahara, H. Taniguchi, S. Yamashita, Y. Nakazawa, M. Maesato and G. Saito.
The Electron Pairing Mechanism of Iron-based Superconductors
This presentation starts with a brief review of superconductors for which evidence for spin-triplet (or pseudo-spin triplet) states have been obtained . Then we examine the available strong evidence for spin-triplet superconductivity of Sr2RuO4 . Next, we focus on two recent results related to the chiral p-wave nature of Sr2RuO4. The first topic is on the observation of half-size magnetization jumps in micron-size Sr2RuO4 rings, ascribable to the formation of half-fluxoid states related to Half-Quantum Vortices (HQV) . The second is on an S/N/S junction formed by depositing a thick film of Pb on the surface of a Sr2RuO4 crystal with eutectic Ru inclusions of micron-size . The junction exhibits an unusual interference between the two superconductors, attributable to the behavior expected for a “topological superconducting junction”.
This presentation is mainly based on the results of collaboration with K. Ishida, T. Nakamura, S. Kittaka, S. Yonezawa, M. Sigrist, J. Jang, R. Budakian, D. G. Ferguson, V. Vakaryuk, S. B. Chung, and P. M. Goldbart.
 for example: H. Tou, K. Ishida and Y. Kitaoka, J. Phys. Soc. Jpn. 74, 1245 (2005).
 A.P. Mackenzie and Y. Maeno, Rev. Mod. Phys. 75, 657 (2003).
 J. Jang et al., Science 331, 186 (2011).
 T. Nakamura et al., preprint.
Reconciliation of quantum oscillations and ARPES in
the underdoped cuprates
The Cuprate Phenomena
The phenomena observed in the Cuprate family of high temperature super-conductors have led to a re-examination of the fundamental concepts with which condensed matter physics has been done for nearly a hundred years. These include the concept of quasi-particles, scaling in critical uctuations between spatial and temporal correlations, conditions for mass generation or gaps in the normal state, and the ratio of the energy scales of collective uctuations driving high temperature superconductivity. The goal of a theory should be that all of these follow from a single paradigm. I will review progress towards this goal by comparison of the results of a systematic theory with experiments.
On superconductivity in the topological insulator Bi2Se3 intercalated by Cu
T.V. Bay, Y.K Huang, H. Luigjes, M.S. Golden and A. de Visser
Topological insulators provide a new tool for the realization of novel states of quantum matter. Recently, it has been recognized that the thermoelectric materials Bi2Se3 and Bi2Te3 are topological insulators: the bulk is insulating, but the surface states - protected by topology - are conducting. Surprisingly, it has been reported recently that these materials can be turned into superconductors by reacting with transition metal elements, like Cu or Pd [1,2]. Notably, Hor and co-workers [1,2] showed that by intercalating Cu into the Van der Waals gaps between the Bi2Se3 layers, superconductivity occurs with a transition temperature Tc = 3.8 K for CuxBi2Se3 with 0.12 x 0.15. However, superconductivity is fragile, as no complete superconducting transition was observed in transport measurements. Moreover, these results have not been confirmed in the literature so far. Here we report our recent efforts to synthesize superconducting CuxBi2Se3 crystals. Several batches of Cu intercalated Bi2Se3 with x = 0.12, 0.14 and 0.15 were prepared and characterized by magnetotransport. All samples showed metallic behaviour. From Hall data we obtained a carrier concentration n 1.41020 cm-3 for x = 0.14. Only the samples rapidly cooled after annealing at 600 C for ~100 hours showed traces of superconductivity. The largest effect was obtained for a sample with x = 0.14 which showed a ~15 % drop in resistance below Tc = 2.3 K. The drop is gradually depressed in a magnetic field, as expected for a superconducting transition. Interestingly, similar transitions with limited resistance drops have been observed in PdzBi2Te3 with 0.15 z 1 . Susceptibility measurements to investigate the diamagnetic screening signal are underway. The results obtained so far indicate the superconducting properties depend sensitively on the sample preparation process. We will compare our data with the recent literature and discuss the possible evidence for bulk superconductivity in CuxBi2Se3.
 Y.S. Hor et al., Phys. Rev. Lett. 104, 057001 (2010)
 Y.S. Hor et al., J. Phys. Chem. Solids, in press; arXiv:1006.0317v1
Type-II Josephson effect
Intermediate-state flux structures in mesoscopic type-I superconductors – effect of sample geometry and applied current
G. R. Berdiyorov,1 A. D. Hernandez-Nieves,1, 2 F. M. Peeters,1 and D. Dominguez2
1 Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
2 Centro Atomico Bariloche, 8400 San Carlos de Bariloche, Rio Negro, Argentina
The effect of sample geometry on the intermediate state (IS) flux structures in mesoscopic type-I superconductors is studied within the Ginzburg-Landau (GL) formalism . In addition to the well-established tubular and laminar structures, the strong confinement leads to the formation of a phase of singly quantized vortices, which is typical for type-II superconductors, and a ring of a normal domain at thermodynamic equilibrium. The stability region and the formation process of these IS patterns are strongly influenced by the geometry of the sample. In samples where the magnetic field is parallel to surfaces (e.g., cubes and disks) the normal domains are mostly located parallel to the surface, whereas in spheres and cones radial distributions of laminar structures are obtained.
We also study the non-linear dynamics of IS flux structures in current-carrying type-I mesoscopic superconductors at zero magnetic field. The results of our numerical simulations within the time-dependent GL theory show that, depending on the applied current, the flux can penetrate the sample in the form of either tubes or stripes (laminae), the shape and the size of them are strongly dependent on the material properties (e.g., the GL parameter), the sample dimensions and the edge imperfections. We also found that the annihilations process of normal domains containing opposite flux takes place in a discrete way releasing single-quantum vortices, which results in measurable traces in the voltage vs. time characteristics of the sample. A phase diagram is calculated which shows the critical thickness of the sample, below which the system exhibits a type-II behavior with single quantized vortices.
 G. R. Berdiyorov, A. D. Hernandez, and F. M. Peeters, Phys. Rev. Lett. 103, 267002 (2009).
Chebyshev-BdG: an efficient method for describing inhomogeneous superconductivity
Inhomogeneities are ubiquitous in real materials, either as impurities, surfaces, interfaces and/or magnetic fields or as quantum or temperature induced fluctuations. We will show how arbitrary inhomogeneous configurations can be efficiently treated numerically by expanding the Gorkov Green's functions of a superconductor in terms of Chebyshev polynomials . Our method is efficient and is amenable to parallel computation on both parallel clusters and graphical processing units. We will show three examples on how the CBdG method can be used efficiently to describe inhomogeneous superconductivity. First we will consider graphene under inhomogeneous strain and show that the superconducting proximity effect in such a material has peculiar properties. It is known that in graphene a specific type of strain will induce pseudo-magnetic fields which in turn induce pseudo-Landau levels . We show that these states have a broken sub-lattice symmetry and that they will have a drastic effect on the Josephson current through such a system. The second example is the study of the superconducting proximity effect and the formation of Andreev levels for a high-Tc superconductor with d-wave symmetric order parameter which undergoes phase fluctuations. We show that the Andreev peaks survive at temperatures higher than Tc which as observed in STM experiments . Third, we will show how impurities affect the superconducting proximity effect for both s-wave and d-wave superconductors. The formation of Andreev levels in metals surrounded by s-wave and d-wave superconductors is considered under both the presence of magnetic fields and random impurities. We show how new states are formed by the interference of quasi-particles scattering of normal/superconductor interfaces and impurities. The same method can be straightforwardly applied to mean-field Hamiltonians with realistic band structures and multi-band superconductivity and in the presence of other order parameters (i.e. anti-ferromagnetic or ferromagnetic).
 L. Covaci, F.M. Peeters and M. Berciu, Phys. Rev. Lett 105, 167006 (2010).
 F. Guinea, M. I. Katsnelson, and A. K. Geim, Nat Phys 6, 30 (2010).
 O. Yuli, I. Asulin, Y. Kalcheim, G. Koren, and O. Millo, Phys.
Rev. Lett. 103, 197003 (2009).
New types of the Fulde-Ferrell-Larkin-Ovchinnikov states induced by anisotropy effects
Dimitry Denisov, A. I. Buzdin
It is well known that in type II superconductors the Abrikosov vortex state can be formed under a magnetic field. In most cases the destruction of superconductivity happens due to the orbital effect. However there can be a situation (for example, in some heavy fermion superconductors) when paramagnetic effect plays an important role in destruction of superconductivity. In this case the nonuniform Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state appears in superconductors, which is characterized by the modulation of the order parameter in real space.
The crystal structure determines both the Fermi surface and pairing symmetry of the superconducting metals. It is demonstrated in the framework of the general phenomenological approach that this is of primary importance for the determination of the structure of the FFLO phase in the magnetic field. The FFLO modulation of the superconducting order parameter may be revealed in the form of the higher Landau-level states or/and modulation along the magnetic field. The transition between different FFLO states could occur with the temperature variation or with the magnetic field rotation. Using numeric calculations we show precise distribution of Landau levels consisting in such states. We found that in the case of higher Landau level solution the superconducting order parameter function has modulation in the direction perpendicular to the applied magnetic field.
Controlling spin motion and interactions in a
one-dimensional Bose gas
Erik van Heumen
The recently discovered iron pnictide superconductors have turned into one of the most widely studied systems in condensed matter physics. We combine low energy electron diffraction (LEED) with simulations to study the cleavage surface of BaFe2-xCoxAs2. We show that it consists of half a Ba layer, while the topmost As-Fe2-As layer is distorted. Including these distortions in density functional calculations we identify several surface related states in the electronic structure measured by ARPES. Further analysis of the ARPES data, comparing to recent dynamical mean field calculations , shows that these materials are strongly correlated.
 A. Kutepov, K. Haule, S. Y. Savrasov, and G. Kotliar, Phys. Rev. B 82, 045105 (2010).
Flux quantization in double layer exciton superfluids
We show that, despite the fact that excitons are charge neutral, the double-layer exciton superfluid exhibits a diamagnetic response. In devices with specific circular geometry, the magnetic-flux threading between the layers must be quantized in units of h/e times χm, where χm is the diamagnetic susceptibility of the device. We discuss possible experimental realizations of the predicted unconventional flux quantization.
Rademaker, Zaanen and Hilgenkamp, PRB 83, 012504 (2011)