Session J21: Focus Session: Search for New Superconductors: New Theories and Experiments

The Challenge of Unconventional Superconductivity

In the past few decades, several new classes of superconductors have been discovered. Most of these are unconventional in that they do not appear to be related to traditional superconductors where the order parameter is more or less spatially isotropic. As a consequence, it is felt by many (but not all!) that the cause of superconductivity arises from a different source than the electron-ion interactions that are at the heart of conventional superconductivity. But developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge, and will continue to be one of the major problems in physics in the decades to come. This is particularly true in that if history is any guide, even more dramatic discoveries of unconventional superconductors await us in the future.   

Transport properties of layered Ba(Pb,Bi)O$_3$ thin films

Doped BaBiO$_3$ is a 3D oxide superconductor with a maximum T$_{\rm c}$ of 30 K for Ba$_{0.6}$K$_{0.4}$BiO$_3$. There has been a lot of discussion on whether this high T$_{\rm c}$ can be explained purely by electron-phonon coupling with a high coupling constant $\lambda$. In addition, the presence of real-space paired $6s^2$ electrons in the parent compound raise intriguing questions about whether there is an electron-electron coupling interaction as well. This possible negative-U interaction might be used to implement the suggestion by Berg, Orgad and Kivelson [Phys.Rev.B 78, 094509] that for a two-layer system where one layer provides electron pairing interaction and the other layer is conducting, the whole can be superconducting with a high T$_{\rm c}$. Here we discuss the transport properties of BaPbO$_3$/BaBiO$_3$ bilayers, where the BaBiO$_3$ layer is thought to act as the pairing layer, while the BaPbO$_3$ acts as the conducting layer. The transport behavior changes to insulating upon decreasing the metallic BaPbO$_3$ layer thickness at values that single films are expected to still be metallic.   

Control of Two-Dimensional Multi-Component Superconductivity in SrTiO$_3$ Heterostructures by Interlayer Coupling

Two-dimensional (2D) superconductivity (SC) in the clean limit is of particular interest since novel phases such as the Fulde-Ferrell-Larkin-Ovchinnikov state [1,2], and Landau-quantized SC [3] are suggested theoretically. In an attempt to approach this limit experimentally, SC was confined in a narrow $\delta$-doped region of SrTiO$_{3}$ [4]. 2D subbands (SBs) were observed as well as 2D SC. Building on these results, we have explored the coupling between two 2D superconducting layers in structures with two $\delta$-doped layers, with variable interlayer coupling controlled by their spacing. We observe multiple components of the SC in the temperature and angular dependent upper critical field plots. These data suggest that the SB structure in the doped and interlayer regions directly impact the SC, and may play an important role for the design and investigation of multi-component SC. [1] P. Fulde $et$ $al$., $Phys$. $Rev$. $\bf 135$, A550 (1964). [2] A. I. Larkin $et$ $al$., $Sov$. $Phys$. $JETP$ $\bf 20$, 762 (1965). [3] M. Rasolt $et$ $al$., $Rev$. $Mod$. $Phys$. $\bf 64$, 709 (1992). [4] Y. Kozuka $et$ $al$., $Nature$ $\bf 462$, 487 (2009).   

Electric field induced superconductivity in a layered transition metal chalcogenide

Recent developments in electric double layer transistors (EDLTs) are attracting growing interests because of its stronger field effect orders of magnitude larger than other transistor techniques This method provides unique abilities to reach the high carrier densities required for inducing superconductivity in several kinds of materials. Among them, layered materials are convenient examples to work with since high quality surface suitable for transistor channel could be easily obtained after mechanical cleavage. Especially, after the introduction of graphene techniques, high quality atomically flat surface can be routinely fabricated on a broad range of layered materials. Combining EDL with novel materials processing techniques on layered materials provides new opportunities in manipulating their electronic properties. We can achieve high carrier density up to 10$^{14}$ cm$^{-2}$ electrostatically in layered materials and induce metal insulator transitions. Superconductivity, similar as that shown in ZrNCl EDL transistor, could be observed when we cool down the system to low temperature after inducing a metal insulator transition with large amount of accumulated carriers. The versatility of this combination shows its potential as a protocol to study varieties of layered materials for broader scope of possibilities in accessing their superconductivities. And hopefully, this method could also facilitate to induce superconductivity in new materials.   

Fano resonances in multigap Fe based superconductors and complexity for material design

The Fano resonance in the superconducting gaps (or ``shape resonance'' or ``Feshabch resonance'' ) in multigap superconductors [A Bianconi \textit{Sol. State Commun.}89, 933 (1994)] has been proposed as the mechanism for high Tc in Fe-based superconductors and related compounds [D Innocenti et al \textit{Supercond. Sci. Technol.}, 015012 (2011)] near the Lifshitz transition for a vanishing Fermi surface in a superlattice of layers or wires, in the proximity of a lattice, electronic, magnetic instability with competing interactions that give complex systems. The multiscale phase separation from nano-scale to micron scale in K0.8Fe1.6Se2. [ A Ricciet al \textit{Phys. Rev.}B 84, 060511 (2011)] has been detected by a mixed real space and momentum space probe: scanning nano focused X-ray diffraction like in La2CuO4+y [M. Fratini, et al \textit{Nature}466, 841 (2010) and [N. Poccia et al \textit{Nature Materials}10, 733 (2011)] showing scale free structural organization of dopants favoring in the high Tc phase. The results on KFeSe show phase separation, percolating superconductivity, competing with percolating magnetism and shape resonances in the superconducting gaps.   

Ordering of dopants and potential increase of Tc to near room temperature

This talk will describe a novel method to increase the resistive Tc of cuprate superconductors to values that might approach room temperature, especially if applied to the underdoped region of the Tc versus carrier concentration phase diagram. This is the part of the so-called pseudogap region that exhibits energy gaps and a small Meissner effect well above the maximum resistive transition [1]. The method proposed here involves ordering of the dopants that provide the itinerant holes in the copper oxygen planes. These dopants also act as pair breakers since they are defects in the structure. The strategy we are proposing here is to separate the regions with dopants which provide the itinerant carriers in the cuprate planes from the metallic but dopant free regions nearby, but far enough away to be not seriously affected by the proximity effect. If this separation can be carried out appropriately and we will describe how this will be done in the talk, there will be fully connected high Tc regions that can fully span a sample and present a very high temperature resistive transition, approaching room temperature for some of the cuprates. \\[4pt] [1] V.Z. Kresin and S.A. Wolf, ArXiv 1109.0341   

Inhomogeneous superconducting state and the intrinsic Tc: Near room temperature superconductivity in the cuprates

Doped cuprates are inhomogeneous superconductors and exhibit properties strongly affected by this inhomogeneity. The notion of an intrinsic critical temperature whose value greatly exceeds the resistive Tc is supported by a number of experimental studies and these will be reviewed in this talk. In particular the anomalous diamagnetism observed above the resistive transition is a manifestation of the presence of superconducting clusters embedded in a normal metallic matrix. The value of intrinsic critical temperature, that reflects the onset of superconductivity in the highest transition temperature clusters, is in some cuprates near room temperature. The transition to the fully superconducting state is percolative in nature and is strongly dependent on the inhomogeneities. Some consequences of such a system, including the ac response will be described.   

A Kohn Luttinger perspective on topological superconductivity

On the basis of an orbital angular momentum and point group symmetry analysis, we argue that electron-driven fluctuations from the viewpoint of a Fermi surface instability generically provide a propensity towards topological superconductivity when the irreducible lattice representations associated with the Cooper pairs are multi-dimensional. For illustration, we explicate our generation recipe of topological superconductivity for the cases of ruthenates, cobaltates, and graphene doped to van Hove filling, i.e. representatives for square, triangular, and honeycomb lattice pairing.   

X-ray photoelectron spectroscopy of Single Crystals of intercalated Bismuth Selenide

The possibility of superconductivity in topological insulators is likely to yield new physics, especially because the insulating normal state is highly unconventional. However, before moving on to microscopic theory, it is important to characterize and study basic systems available today. Here, we present XPS studies of intercalated single crystals of Bi$_{2}$Se$_{3}$ and Sb$_{2}$Se$_{3}$. Core level spectroscopy, combined with infra-red and Raman spectroscopy, yield details of the nature of bonding of atoms at the cleaved surface.   

Search for Superconductivity in Single Crystals of Topological Insulators

Observation of superconductivity in Cu-intercalated Bi$_{2}$Se$_{3}$, and in Bi$_{2}$Te$_{3}$ under pressure, open up the possibility of occurrence of superconductivity in other similarly modified structures. Here, we present detailed studies from a comprehensive search for superconductivity in selenides and tellurides of Bismuth and Antimony. High-quality single crystals were intercalated with a number of different elements using both in situ and ex situ techniques, then scanned for a resistive or magnetic transition to superconductivity. With an intent to understand the nature of intercalation in these compounds, we discuss results from our search for superconductivity, together with crystal structure analysis and detailed optical and x-ray photoelectron spectroscopy studies of the cleaved surface.   

Surface Andreev Bound States of Topological Superconducting Phase in Doped Semiconductors: Application to Cu$_x$Bi$_2$Se$_3$

The recently discovered superconductor Cu$_x$Bi$_2$Se$_3$ is a candidate for three-dimensional time-reversal-invariant topological superconductors, which is predicted to have robust surface Andreev bound states hosting massless Majorana fermions. In this work, we present an analytical and numerical study of the surface Andreev bound state wavefunction and dispersion. We find the topologically protected Majorana fermions at $k=0$, as well as a new type of surface Andreev bound states at finite $k$. We relate our results to a recent point-contact spectroscopy experiment.   

Functional renormalization group and variational Monte Carlo studies of the electronic instabilities in graphene near 1/4 doping

We study the electronic instabilities of near 1/4 electron doped graphene using the functional renormalization group (FRG) and variational Monte-Carlo method. A modified FRG implementation is utilized to improve the treatment of the von Hove singularity. At 1/4 doping the system is a chiral spin density wave state exhibiting the anomalous quantized Hall effect, or equivalently a Chern insulator. When the doping drops below 1/4, the $d_{x^2-y^2}+i d_{xy}$ Cooper pairing becomes the leading instability. Our results suggest near 1/4 electron- or hole-doped graphene is a fertile playground for the search of Chern insulators and superconductors.