# Multicomponent and Strongly Correlated Superconductivity

Europe/Stockholm
FB52 (AlbaNova)

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Description

This conference is a satellite of the 28th International Conference on Low Temperature Physics, LT28, www.lt28.se, held 9-16 August, 2017.

### Venue

Nordita, Stockholm, Sweden

Modern condensed matter systems of interest, including those which display the highest superconducting transition temperatures, tend to exhibit a complex interplay of many-body ordering tendencies with ensuing multicomponent field theories. Understanding the microscopic origin and the phenomenology of such interplay has been among the most active frontiers of research in recent years. The conference will cover a wide range of topics such as: superconductivity in iron-pnictides, cuprates, and in materials without inversion symmetry, as well as other systems with unconventional pairing symmetry.

### Organizers

Egor Babaev
Karen Michaeli
Mihail Silaev

Daniel Agterberg
Asle Sudbo
Oskar Vafek

• Thursday, 17 August
• 09:00 09:35
Daniel Agterberg - Superconductivity and magnetism in monolayer FeSe 35m FB52

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Superconductivty in monolayer $FeSe$ grown on $SrTiO_3$ represents a new example of a correlated material with high- Tc superconductivity. The origin of superconductivity and the electronic structure of normal state remain under debate. I will present recent results on developing a realistic model for the normal state and its interplay with magnetic fluctuations. Two key inputs are: the unavoidable multiple-orbital nature of the relevant bands; and, as indicated by density functional theory, the role of fluctuations stemming from a translation invariant magnetic order.
• 09:35 10:10
Bernd Buchner - Nematic order and orbitals in Fe-based superconductors 35m FB52

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• 10:10 10:40
Coffee break 30m AlbaNova entrance

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• 10:40 11:15
Ophir Auslaender - Local measurement of the superfluid density in several iron-based superconductors 35m FB52

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We have used magnetic force microscopy (MFM) to characterize superconductivity locally across the superconducting dome in the electron, hole, and isovalently doped iron pnictides [$Ba_{1-x}K_xFe_2As_2, Ba(Fe_{1- x}Co_x)_2As_2, BaFe_2(As_{1-x}P_x)_2$]. We find a sharp increase of the absolute value of the penetration depth (?ab) upon approaching the underdoped edge of the superconducting dome, which suggests the superfluid density $n_s=1/\lambda_{ab}^2$ vanishes. On the overdoped side $\lambda_{ab}$ flattens out. The behavior around optimal doping depends on the material. In $Ba(Fe_{1- x}Co_x)_2As_2$ we observe a weak minimum of  $\lambda_{ab}$. In $BaFe_2(As_{1-x}P_x)_2$ we find a clear maximum and in $Ba_{1-x}K_xFe_2As_2$ what appears to be an abrupt jump in  $\lambda_{ab}$. This hints at the possible existence of a phase that gives rise to mass renormalization. In all samples except underdoped ones superconductivity is homogenous, even around optimal doping. Very underdoped samples are less homogeneous because of local variations of doping. In mildly underdoped $BaFe_2(As_{1-x}P_x)_2$ we observe stripes of enhanced diamagnetic response.
• 11:15 11:50
Henning Klauss - Universal suppression of the magnetic order parameter in 122 Fe-based superconductors 35m FB52

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The interplay of itinerant magnetism, electronic nematic order and unconventional superconductivity in Fe based superconductors and other intermetallic systems with complex Fermi surfaces is a fascinating topic in contemporary correlated electron physics. In my talk I will discuss Mössbauer spectroscopy and muon spin relaxation experiments on several series of hole doped 122 pnictides like $Ba_{1?x}K_xFe_2As_2$$^{1,2}$) and $Ca_{1?x}Na_xFe_2As_2$. The parent compound $CaFe_2As_2$ of the latter series shows a spin density wave order below TN = 165 K 3). Our microscopic study proves that with increasing Na-substitution level, the magnetic order parameter as well as the magneto-structural phase transition is suppressed. For $x = 0.50$ we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature TC. A systematic comparison with other 122 pnictides reveals a linear correlation between the magnetic order parameter reduction and the ratio of the transition temperatures, TC/TN, which can be understood in the framework of a Landau- theory$^3$). In the optimally doped specimen with $T_c \approx 34 K$, the temperature dependence of the penetration depth and superfluid density were obtained, which proves the presence of two superconducting s-wave gaps. 1) E Wiesenmayer - Phys. Rev. Lett. 107, 237001 (2011) 2) T Goltz et al - Phys. Rev. B 89, 144511 (2014) 3) P. Materne et al. - Phys. Rev. B 92, 134511 (2015)
• 11:50 12:25
Guido Pupillo - Glass and superglass phases of monodispersed ultrasoft particles 35m FB52

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We investigate the quantum phases of monodispersed classical and quantum bosonic gases confined to two dimensions and interacting via a class of soft-shoulder potentials. The latter correspond to soft-core potentials with an additional hard-core onsite interaction, and are relevant both to superconducting materials with multi-scale intervortex forces and cold atomic gases. Using a combination of classical molecular dynamics simulations in free space and exact quantum Monte Carlo simulations on a lattice, we show that the low temperature phases for weak and strong interactions following a temperature quench are a homogeneous (super)fluid and a glass, respectively. The latter is an insulating phase characterized by inhomogeneity in the density distribution and structural disorder. Remarkably, we find that for intermediate interaction strengths a "superglass" occurs in an extended region of the quantum phase diagram, where glassy behavior coexists with a sizable finite superfluid fraction. This glass phase is obtained in the absence of geometrical frustration or external disorder and is a result of the competition of quantum fluctuations and cluster formation in the corresponding classical ground state. Given the simplicity and generality of the model, this superglass phases should be directly relevant for state-of-the-art experiments with Rydberg-dressed atoms in optical lattices.
• 12:25 14:30
Lunch 2h 5m AlbaNova restaurant

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• 14:30 15:05
Kamran Behnia - Superconductivity and ferroelectricity in strontium titanate 35m FB52

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The large-gap semiconductor strontium titanate ($SrTiO_3$) becomes a metal upon removal of a tiny fraction of its oxygen atoms. The dilute metal has a sharp Fermi surface and is subject to a superconducting instability. Discovered half-a- century ago, the superconducting dome of strontium titanate remains doubly mysterious: How can superconductivity persist when there is only one carrier for $10^5$ atoms and the Fermi energy an order of magnitude smaller then than the Debye energy? What destroys this cooperative order as soon as carrier density exceeds 0.02 electrons per formula unit? On the other hand, substituting strontium with calcium stabilizes a long-range ferroelectric order in $Sr_{1- x}Ca_xTiO_3$. We find that in $Sr_{1-x}Ca_xTiO_{3- \delta}$ ferroelectricity coexists with metallicity and its superconducting instability in a narrow window of doping. As the carrier concentration is increased, the ferroelectric order is eventually destroyed by a quantum phase transition. This happens at a critical doping level at which the Friedel oscillations generated by neighboring dipoles interfere destructively. In the vicinity of this quantum phase transition, the superconducting critical temperature is enhanced. We will discuss a possible link to ferroelectric quantum criticality.
• 15:05 15:40
Ilya Ermin - s + is superconductivity with incipient bands: doping dependence and STM features 35m FB52

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Motivated by the recent observations of small Fermi energies and comparatively large superconducting gaps, present also on bands not crossing the Fermi energy incipient bands in iron-based superconductors, we analyse the doping evolution of superconductivity in a four-band model across the Lifshitz transition including BCS-BEC crossover effects on the shallow bands. Similar to the BCS case we find that with hole doping the phase difference between superconducting order parameters of the hole bands changes from $0$ to $\pi$ through an intermediate $s+is$ state breaking time-reversal symmetry. The transition however occurs in the region where electron bands are incipient and chemical potential renormalization in the superconducting state leads to a significant broadening of the $s+is$ region. Our results are quite actual given that the signatures of the s+is state were found recently by means of muSR experiments in Ba122 doped with K. Furthermore, we present the qualitative features of the $s+is$ state that can be observed in scanning tunnelling microscopy (STM) experiments using the recently proposed method.
• 15:40 16:10
Coffee break 30m AlbaNova entrance

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• 16:10 16:45
Clifford Hicks - Strain-Tuning of the Superconductor SrRuO 35m FB52

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The superconductivity of $Sr_2RuO_4$, suspected to have a chiral order parameter $\Delta\sim p_x\pm ip_y$, responds sensitively to uniaxial pressure. In this talk, I will present evidence that uniaxial pressure drives one of the Fermi surfaces through a Van Hove singularity at a compression of $\sim0.6\%$, and at this point the superconductivity is dramatically strengthened: Tc passes through a pronounced peak, at which it is more than double its zero-pressure value, and $Hc_2$ is enhanced by more than a factor of twenty. This very strong enhancement of $Hc_2$ suggests even- parity pairing at this strain, meaning that there might be a strain-induced odd-parity to even-parity transition.
• 16:45 17:20
Carsten Timm - Inflated nodes and surface states in multiband super- conductors with broken time-reversal symmetry 35m FB52

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It is commonly believed that superconductors fall into one of three classes: They can have a full energy gap, a gap with point nodes, or a gap with line nodes. We have shown in [1] that multiband, even-parity, nodal superconducting states that break time-reversal symmetry do not belong to these classes. Instead, they generically possess two-dimensional Fermi surfaces. These Fermi surfaces are generated by ? inflating? point and line nodes into spheroidal and toroidal pockets, respectively, by a pseudomagnetic field resulting from interband pairing. Such states can be energetically stable in spite of the extended Fermi surfaces. The inflated nodes are topologically protected by a Z2 invariant, which we give in terms of a Pfaffian [1]. In addition, inflated point nodes retain a nontrivial Chern number, while inflated line nodes are characterized by a second Pfaffian. Finally, I will discuss the surface states and their interplay with the inflated nodes in multiband superconductors. [1] D. F. Agterberg, P. M. R. Brydon, and C. Timm, Phys. Rev. Lett. 118, 127001 (2017).
• 17:20 17:55
Konstantin Efetov - Charge and Current Modulations in a Spin-Fermion model with overlapping hotspots and physics of cuprates 35m FB52

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Several well-known phenomena in the hole-doped cuprates like breaking the rotational invariance, appearance of the pseudogap, charge modulation and d-wave superconductivity occur on a low energy scale of hundreds Kelvin. As it is not quite clear how to obtain these phases in an unified way from microscopic models of cuprates, we consider a low-energy model of fermions interacting with close to critical antiferromagnetic excitations. In contrast to a standard spin- fermion model, we assume in agreement with ARPES data that the fermion spectrum in the antinodal region is shallow, such that the 8 hotspots merge at not very weak interaction into 2 antinodal hot regions. In addition to the interaction via antiferromagnetic fluctuations, a long range part of the Coulomb interaction reducing the superconducting transition temperature is taken into the consideration. It is demonstrated in the mean field approximation that a variety of phase transitions are possible depending on the chemical potential and details of the electronic spectrum near the antinodes. In addition to the d-wave superconductivity and charge density wave with the diagonal modulation, we find a nematic transition (Pomeranchuk instability) followed by a transition to a charge density wave with a modulation along the bonds and d-wave formfactor. Moreover, it is found that an electron-hole pairing with a vector connecting to neighboring antinodes (antiferromagnetic vector of cuprates) is also possible. Remarkably, this pairing leads to circulating currents rather than to a charge modulation. These currents are similar to those proposed in DDW (d-density wave state). Depending on the parameters of the electron spectrum one can also obtain an incommensurate structure of circulating currents. The nematic transition does not lead to formation of the gap but the circulating currents do. This gap is located at the antinodes and we associate this state with the pseudogap state. The results of our theory can serve as an explanation of recent experiments on cuprates performed with the help of STM, NMR, hard and resonant soft X-ray scattering, neutron scattering, sound propagation, and with some other techniques.
• Friday, 18 August
• 09:35 10:10
Vlad Kozii - Three-Dimensional Majorana Fermions in Chiral Superconductors 35m FB52

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Through a systematic symmetry and topology analysis we establish that three-dimensional chiral superconductors with strong spin-orbit coupling and odd-parity pairing generically host low-energy nodal quasiparticles that are spin-non- degenerate and realize Majorana fermions in three dimensions. By examining all types of chiral Cooper pairs with total angular momentum J formed by Bloch electrons with angular momentum j in crystals, we obtain a comprehensive classification of gapless Majorana quasiparticles in terms of energy-momentum relation and location on the Fermi surface. We show that the existence of bulk Majorana fermions in the vicinity of spin-selective point nodes is rooted in the non-unitary nature of chiral pairing in spin-orbit- coupled superconductors. We address experimental signatures of Majorana fermions, and find that the nuclear magnetic resonance spin relaxation rate is significantly suppressed for nuclear spins polarized along the nodal direction as a consequence of the spin-selective Majorana nature of nodal quasiparticles. Furthermore, Majorana nodes in the bulk have nontrivial topology and imply the presence of Majorana bound states on the surface that form arcs in momentum space. We conclude by proposing the heavy fermion superconductor PrOs4Sb12 and related materials as promising candidates for non-unitary chiral superconductors hosting three-dimensional Majorana fermions.
• 10:10 10:40
Coffee break 30m AlbaNova entrance

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• 10:40 11:15
Eun-Ah Kim - Let there be topological superconductors 35m FB52

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Superconductivity research has traditionally been discovery driven. Of course $T_c$ is a non-universal quantity that cannot be predicted, hence off-limits to theorists. Nevertheless, it must be possible to reach intelligent predictions for superconductors that are interesting for reasons other than high $T_c$ per se. Of particular interest are topological superconductors under pursuit as a platform for quantum computing. I will present our recent results proposing two different platforms for topological superconductivity: p-doped transition metal dichalcogenides (TMD?s) and quantum spin-ice/metal heterostructure. For the p-doped TMD?s the strategy is to break the spin-degeneracy in momentum space. For the quantum spin-ice/metal the strategy is to let the spin fluctuation of quantum spin-ice mediate pairing in the metallic layer. I will close the talk with experimental implications and prospects for realizing the proposed topological superconductors.
• 11:15 11:50
Vladimir Juricic - Superconducting quantum criticality and competing states in graphene-like materials 35m FB52

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• 11:50 12:25
Alexander Zyuzin - Nematic skyrmions in odd-parity superconductors 35m FB52

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• 12:25 14:30
Lunch 2h 5m AlbaNova restaurant

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• 14:30 15:05
Egor Babaev - s+is superconducting state: Mixed collective modes, do- main walls, skyrmions, and generic breakdown of type- I/type-II dychotomy 35m FB52

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• 15:05 15:40
Masaki Oshikawa - TBA 35m FB52

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• 15:40 16:10
Coffee break 30m AlbaNova entrance

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• 16:10 16:45
Vadim Grinenko - Superconductivity with broken time reversal symmetry in BaKFAs single crystals 35m FB52

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• Saturday, 19 August
• 09:00 09:35
Jeremy Levy - Intrinsic One-Dimensional Nature of Pairing and Supercon- ductivity at the LaAlO/SrTiO Interface 35m FB52

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$SrTiO_3$ is a superconducting semiconductor whose pairing mechanism has remained mysterious for more than half a century. The development of $SrTiO_3$-based heterostructures and nanostructures has enabled systematic investigation of the role of dimensionality on superconducting behavior. Here, we examine superconductivity in a family of $LaAlO_3/SrTiO_3$ channels whose widths systematically traverse the 1D-2D transition. The superconducting critical current of $LaAlO_3/SrTiO_3$ is found to be independent of width ranging from 10-1000 nm; the narrower channels exhibit densities that are at least an order of magnitude larger than for macroscopic 2D $LaAlO_3/SrTiO_3$ interfaces. Increasing the number of 1D channels results in a marked increase in critical current and suppression of background resistance. Resistive anomalies outside of the superconducting regime are observed for the narrow channels, and are attributed to a state in which electrons are paired but not superconducting. These results strongly indicate that electron pairing?and superconductivity?exist only along the boundaries of these channels and are absent within the interior region of the channels. These experiments provide new physical constraints on possible pairing mechanisms for $SrTiO_3$-based systems.
• 09:35 10:10
Maxim Khodas - Interplay of magnetism superconductivity and orbital order in iron bases superconductors 35m FB52

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In this talk I will present the theory of interplay between different macroscopic orderings universally found in iron based superconductors. In most of iron pnictides nematic order is a byproduct of stripe magnetism. In FeSe, the nematic transition is not followed by magnetism at ambient conditions. I will present a unifying description of both of these two different scenarios. Next I'll present the parquet Renormalization Group (RG) study of the interplay between different ordering tendencies. The RG flow of susceptibilities favors the nematic instability even though the magnetism is the only interaction channel with attraction at the bare level. In most of iron pnictides the Fermi energy is large and RG is not capable of tipping the scale in favour of structural transition. Hence, the leading instability is magnetic or superconducting. In FeSe the small Fermi energy allows for RG flow to run long enough to promote the nematic instability, and at the same time to suppress magnetism.
• 10:10 10:40
Coffee break 30m AlbaNova entrance

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• 10:40 11:15
Morten Eskildsen - Direct Evidence for an Internal Degree of Freedom and Bro- ken Time Reversal Symmetry in the B-phase of UPt 35m FB52

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• 11:15 11:50
Klaus Hasselbach - Magnetic fields above unconventional superconductors 35m FB52

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We will present scanning superconducting quantum interference device (SQUID) microscopy [1] and its contribution to the study of the vortex phases in conventional an unconventional superconductors (Sr2RuO4 and UCoGe).  For the case of UCoGe we will show how coexisting states of superconductivity and magnetism interact with each other by local measurements of the magnetic fields, with micron scale spatial resolution above a single crystal of the ferromagnetic superconductor UCoGe using scanning microSQUID microscopy. Our measurements show that the spontaneous ferromagnetic transition at TC =2.5K [2,3] is characterized by Ising- like magnetization along the easy axis (c direction), with domain sizes of the order of 10 ?m and magnetization amplitudes of 45 G, and are consistent with estimates of domain-wall widths of several angstroms. The measured magnetization amplitudes are in agreement with bulk magnetization measurements. In the superconducting state, which coexists with ferromagnetism below Tsc =0.67K, both diamagnetic screening and Meissner expulsion of flux, but no shrinkage of the ferromagnetic domains, are detected. Although we could not resolve individual vortices, our measurements provide evidence for the existence of the spontaneous vortex state in UCoGe. References [1] JLTP  175, 861, (2014) [2] Phys. Rev. Lett. 109, 237001 (2012) [3] Phys.Rev. B 90, 184501 (2014)
• 11:50 12:25
Atsutaka Maeda - Flux Flow of Iron-based Superconductors - Novel Gap Spectroscopy and Universal large Dissipation 35m FB52

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We investigated superfluid density (penetration depth) as a function of temperature and flux flow resistivity as a function of magnetic field of various kinds of Fe based superconductor systematically by microwave conductivity measurement techniques[1-5]. Reflecting the multiply gapped nature of these materials, large variety of the phenomena was observed both in the temperature dependence of the penetration depth and in the magnetic field dependence of flux flow resistivity. We developed a model that describes the superfluid density and the flux flow resistivity for a two gap superconductor, which take the Fermi surface structure explicitly into account[5]. With available data of the Fermi surface measured by ARPES experiments, we succeeded in explaining the observed behaviors of these two independent quantities QUANTITATIVELY very well in terms of the two band model. Depending on the magnitude of the obtained anisotropy parameters, we confirmed the presence/the absence of the nodes on each Fermi surface. Thus, we can determine the superconducting gap structure investigating these two quantities in detail. Therefore, our method can be called as a novel method to discuss the structure of the superconducting order parameter. In Fe(Se,Te), the dissipation by the flux flow was found to be exceptionally small, which turn out to be the result of the backflow of supercurrent by the disorder specific to this system[4]. Another interesting aspect is, in all materials investigated, the quasiparticle scattering time in the vortex core is rather short so that the mean free path of the quasiparticle in the vortex core is limited by the core radius. Indeed, we already obtained essentially the same features in many other superconductors, which cannot be explained by any existing theories, and may suggest the presence of a novel mechanism of dissipation by quasiparticles in the vortex core. [1] T. Okada et al., Phys. Rev. B86 (2012) 064516. [2] H. Takahashi et al., Phys. Rev. B86 (2012) 144525. [3] T. Okada et al., Physica C484 (2013) 27, ibid C494 (2013) 109, ibid C504 (2014) 24. [4] T. Okada et al., Phys. Rev. B91 (2015) 054510. [5] A. Maeda et al., Quantum Matt. 4 (2015) 308, and T. Okada, in preparation.
• 12:25 15:35
Lunch & Poster Session 3h 10m Lunch outside Albanova restaurant / Posters at Albanova entrance

### Lunch outside Albanova restaurant / Posters at Albanova entrance

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• 15:35 16:10
Peter Hirschfeld - Orbital selective pairing in Fe-based superconductors 35m FB52

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I discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. This approach provides a crude way to incorporate the reduced coherence of quasiparticle occupying specific orbital states via suppression of pair scattering processes involving those less coherent states. I show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated iron-based superconductors.
• 16:10 16:45
Qimiao Si - Orbital-selective effects in iron-based superconductors 35m FB52

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In this talk, I will present an introductory discussion on the multi-orbital physics in the iron-based superconductors. In the normal state, this concerns the effect of electron correlations [1]. I will describe how an orbital-selective Mott phase anchors the orbital selectivity [2], which has been a subject of extensive theoretical and experimental studies. In the superconducting state, I will address how orbital selectivity not only can influence the usual superconducting properties (for instance, gap anisotropy) but also may give rise to new types of orbital-selective superconducting state. The latter is exemplified by a proposed ?s-tau3? pairing state, which provides a natural understanding of some seemingly contradictory properties observed in the iron- chalcogenide superconductors [3]. [1] Q. Si, R. Yu and E. Abrahams, "High Temperature Superconductivity in Iron Pnictides and Chalcogenides", Nat. Rev. Mater. 1, 16017 (2016). [2] R. Yu and Q. Si, "Orbital-selective Mott phase in multiorbital models for iron pnictides and chalcogenides", arXiv:1705.04541. [3] E. M. Nica, R. Yu and Q. Si, "Orbital selective pairing and superconductivity in iron selenides", Npj Quantum Materials, in press; arXiv:1703.03170 (arXiv:1505.04170).
• 16:45 17:20
Girsh Blumberg - Critical Charge Fluctuations in Iron Pnictide Superconductors 35m FB52

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The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low doping concentrations, the tetragonal-to-orthorhombic structural transition is closely followed by a concomitant spin density wave transition both being in close proximity to the superconducting phase. A key question is the microscopic mechanism of high-Tc superconductivity and its relation to multi-orbital ordering and magnetism. Here we study the 111, 122 and 11 families of iron superconductors using low energy polarization resolved Raman spectroscopy. The Raman susceptibility shows critical non-symmetric charge fluctuations across the entire phase diagram. The charge fluctuations are interpreted in terms of plasma waves of quadrupole intra-orbital excitations in which the electron and hole Fermi surfaces breathe in-phase. We demonstrate that above the structural phase transition these quadrupolar fluctuations with long correlation times are precursor to the discrete four-fold symmetry breaking transition. This is manifested in the critical slowing down of XY-symmetry collective fluctuations observed in dynamical Raman susceptibility and strong enhancement of the static Raman susceptibility. Below superconducting transition, these collective excitations undergo a metamorphosis into a coherent in-gap collective mode of extraordinary strength and at the same time may serve as glue for non-conventional superconducting pairing. [1] V. K. Thorsmølle, M. Khodas, Z. P. Yin, C. Zhang, S. V. Carr, Pengcheng Dai, G. Blumberg. Critical Charge Fluctuations in Iron Pnictide Superconductors. Phys. Rev. B 93, 054515 (2016). [2] S.-F. Wu, P. Richard, H. Ding, H.-H. Wen, Guotai Tan, Meng Wang, Chenglin Zhang, Pengcheng Dai, G. Blumberg. Superconductivity and electronic fluctuations in Ba1? xKxFe2As2 studied by Raman scattering. Phys. Rev. B 95, 085125 (2017).
• Sunday, 20 August
• 09:00 09:35
Jeroen van den Brink - Josephson Currents Induced by the Witten Effect 35m FB52

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We point out the existence of a new type of topological Josephson effect involving type II superconductors and three- dimensional topological insulators as tunnel junctions. We predict that vortex lines induce a variant of the Witten effect that is the consequence of the axion electromagnetic response of the topological insulator: at the interface of the junction each flux quantum attains a fractional electrical charge of e/4. As a consequence, if an external magnetic field is applied perpendicular to the junction, the Witten effect induces an ac Josephson effect in the absence of any external voltage. We derive a number of further experimental consequences and propose potential setups where these quantized, flux induced Witten effects may be observed [1]. [1] Physical Review Letters 117, 167002 (2016)
• 09:35 10:10
Lode Pollet - The anisotropic Harper-Hofstadter-Mott model: competi- tion between condensation and magnetic fields 35m FB52

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We derive the reciprocal cluster mean-field method to study the strongly-interacting bosonic Harper-Hofstadter-Mott model. In terms of the hopping anisotropy and the chemical potential, the system exhibits a rich phase diagram featuring band insulating, striped superfluid, and supersolid phases. Furthermore, for finite anisotropy we observe gapless uncondensed liquid phases at integer fillings, which are analyzed by exact diagonalization. The liquid phases at fillings ? = 1, 3 exhibit the same band fillings as the fermionic integer quantum Hall effect, while the phase at ? = 2 is CT - symmetric with zero charge response. We discuss how these phases become gapped on a quasi-one-dimensional cylinder, leading to a quantized Hall response, which is characterized by a suitable measure for non-trivial many-body topological properties. Incompressible metastable states at fractional filling are also observed, indicating competing fractional quantum Hall phases. The combination of reciprocal cluster mean-field and exact diagonalization yields a promising method to analyze the properties of bosonic lattice systems with non-trivial unit cells in the thermodynamic limit.
• 10:10 10:40
Coffee break 30m AlbaNova entrance

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• 10:40 11:15
Evgeny Kozik - Superfluid states and physical nature of the pseudogap in the 2D Hubbard model 35m FB52

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A significant part of the phase diagram of the 2D fermionic Hubbard model in the non-perturbative regime of moderate interactions and filling factors (U<4,n<0.7) is governed by effective Fermi liquid physics with emergent weak BCS-type instabilities. I will discuss how the bold-line diagrammatic Monte Carlo technique with semi-analytic treatment of the Cooper instability can be used to address this system without uncontrolled approximations. I will present an accurate ground-state phase diagram in the (n,U) plane, describing competition between the p- and d-wave superfluid states. However, critical temperatures prove to be small in this regime at least up to U=4, n=0.6, where skeleton expansions for the Hubbard model break down and the bare series diverges. I will introduce a method for controlling the convergence radius of bare diagrammatic expansions, which allows us to investigate in a controllable manner the pseudogap regime of the Hubbard model and to study the nodal/antinodal dichotomy at low doping and values of coupling inaccessible before.
• 11:15 11:50
Emanuel Gull - Simulation of the NMR response in the pseudogap regime of the cuprates 35m FB52

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The pseudogap in the cuprate high-temperature superconductors was discovered as a suppression of the Knight shift and spin relaxation time measured in nuclear magnetic resonance (NMR) experiments. However, theoretical understanding of this suppression in terms of the magnetic susceptiblility of correlated itinerant fermion systems was so far lacking. We will present results of a study of the temperature and doping evolution of these quantities on the two-dimensional Hubbard model using cluster dynamical mean field theory. We recover the suppression of the Knight shift and the linear-in-T spin echo decay that increases with doping. The relaxation rate shows a marked increase as T is lowered but no indication of a pseudogap on the Cu site, and a clear downturn on the O site, consistent with experimental results on single layer materials but different from double layer materials. The consistency of these results with experiment suggests that the pseudogap is well described by strong short-range correlation effects.
• 11:50 12:25
Johan Carlstrom - TBA 35m FB52

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• 12:25 14:30
Lunch 2h 5m outside Albanova restaurant

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• 14:30 15:05
Aline Ramires - The Concept of Superconducting Fitness 35m FB52

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In this talk I introduce a general scheme to probe the compatibility of arbitrary pairing states with a given normal state Hamiltonian by the introduction of a concept called “Superconducting Fitness”. This quantity gives a direct measure of the suppression of the superconducting critical temperature in the presence of key symmetry-breaking fields. A merit of the superconducting fitness is that it can be used as a tool to identify nontrivial mechanisms to suppress superconductivity under various external influences, in particular, magnetic fields or distortions, even in complex multiorbital systems. This concept can also be used in order to favour unusual superconducting order parameters by engineering the normal state Hamiltonian. I discuss the application of this idea to Sr2Ru04, Fe-based superconductors and CePt3Si.
• 15:05 15:40
Flavio Nogueira - Duality between a three-dimensional topological Mott insulator and a topological superconductor 35m FB52

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#### AlbaNova

We consider a compact abelian Higgs model in 3+1 dimensions with a topological axion term and construct its dual theories for both bulk and boundary at strong coupling. The model may be viewed as describing a superconductor with magnetic monopoles, which can also be interpreted as a field theory of a topological Mott insulator. We show that this model is dual to a non-compact topological field theory of particles and vortices. It has exactly the same form of a model for superconducting cosmic strings with an axion term. We consider the duality of the boundary field theory at strong coupling and show that in this case ? is quantized as ?8?n/m where n and m are the quantum numbers associated to electric and magnetic charges. These topological states lack a non-interacting equivalent. Ref.: F. S. Nogueira, Z. Nussinov, and J. van den Brink, Phys. Rev. D 94, 085003 (2016).
• 15:40 16:10
Coffee break 30m AlbaNova entrance

### AlbaNova entrance

#### AlbaNova

• 16:10 16:45
Roman Lutchyn - Coulomb blockade effect in proximitized nanowires 35m FB52

### FB52

#### AlbaNova

Motivated by recent experiments, we consider a mesoscopic island consisting of a proximitized nanowire which is connected via point contacts to normal-state leads. We study transport properties of such islands in the Coulomb blockade regime as a function of an applied magnetic field. In the presence of Rashba spin-orbit coupling, magnetic field can drive a proximitized nanowire into a topological superconducting phase. The associated with the topological superconductivity Majorana modes significantly modify transport and lead to single-electron coherent transmission through the nanowire - a non-local signature of topological superconductivity. In this work, we focus on the case of strong hybridization of the Majorana modes with the normal leads. The induced by hybridization broadening of the Majorana zero-energy states competes with the charging energy, leading to a considerable modification of the Coulomb blockade in a nanowire contacted by two normal leads. We evaluate the two-terminal conductance as a function of the gate voltage, junctions transmission coefficients, the geometric capacitance of and the induced superconducting gap in the nanowire.
• Monday, 21 August
• 09:00 09:35
Thomas Scaffidi - Unconventional Superconductivity in SrRuO 35m FB52

### FB52

#### AlbaNova

$Sr_2RuO_4$ is one of the very few candidates for $p+ip$ superconductivity. Besides the fundamental importance of firmly establishing the existence of a spin-triplet-paired superconductor, the interest in $Sr_2RuO_4$ also emanates from the possibility of half-quantum vortices that host Majorana bound states.In this talk, I will review the progress recently made in the understanding of this material. Using a spin-orbit-coupled weak-coupling calculation, I will analyze the results of several recent experiments: the absence of measurable edge currents observed by magnetometry, the absence of cusp of $T_c$ versus uniaxial strain, and the nodal behavior observed for example in the specific heat. I will show that, in each case, the results do not invalidate the presence of a $p+ip$ state. I will also discuss the evolution of the superconducting state at a Van Hove singularity, which was recently reached experimentally by applying uniaxial strain.
• 09:35 10:10
Adrien Bouhon - Topological classification of nodal band structures (from the normal state to the superconducting state) 35m FB52

### FB52

#### AlbaNova

• 10:10 10:40
Coffee break 30m AlbaNova entrance

### AlbaNova entrance

#### AlbaNova

• 10:40 11:15
Sungkit Yip - p-wave superconductor and superfluid under mesoscopic confinement 35m FB52

### FB52

#### AlbaNova

• 11:15 11:50
Kirill Samokhin - Superconductivity in quantum wires: magnetic symmetry, boundary modes, and fluctuations 35m FB52

### FB52

#### AlbaNova

We present some recent results in the theory of quasi-1D noncentrosymmetric superconductors, in particular, the peculiar features of the gap symmetry in the presence of external magnetic field and also the effects of fluctuations on the Andreev boundary modes.
• 11:50 12:25
Mats Granath - Exploring an effective model for finite momentum paring states 35m FB52

### FB52

#### AlbaNova

We discuss an extension of the standard effective model of d- wave superconductivity of a single band tight-binding Hamiltonian with nearest neighbor attraction to include finite range periodically modulated pair-hopping. The pair-hopping is characterized by a fixed wave number $\vec{Q}$ breaking lattice rotational but not translational symmetry, assuming an underlying nematic order. Within self-consistent BCS theory we study the general variational state consisting of two incommensurate singlet pair amplitude order parameters $\Delta_{\vec{Q}_1}$ and $\Delta_{\vec{Q}_2}$. We find two types of near degenerate ground states; of the Larkin- Ovchnnikov (LO) type with $\Delta_{\vec{Q}_1}=\Delta_{\vec{Q}_2}$ and $\vec{Q}_1=-\vec{Q}_2\approx \vec{Q}$ or of the Fulde- Ferrell (FF) type with $\Delta_{\vec{Q}_2}=0$ and $\vec{Q}_1\approx \pm\vec{Q}$. An anomalous term in the current operator arising from the pair-hopping ensures that Blochs's theorem on zero current in the ground state is enforced also for the FF ground state, despite the spontaneously broken time-reversal symmetry. We also discuss states with a uniform current by exploring the space of pair-momenta $\vec{Q}_1$ and $\vec{Q}_2$. We find a rich phenomenology including the possibility of inhomogeneous current densities due to phase separation and unconventional directional dependence of the depairing current.
• 12:25 14:30
Lunch 2h 5m AlbaNova restaurant

### AlbaNova restaurant

#### AlbaNova

• 14:30 15:05
Karen Michaeli - Electron teleportation in multi-terminal Majorana islands: Kondo effect at high temperature 35m FB52

### FB52

#### AlbaNova

Recent advances in growing complex structures of topological superconducting nanowires have paved the way for exploring new physics beyond the immediate application of finding Majorana modes. In our work we studied setups involving a superconducting island with multiple Majorana modes connected to normal leads. I will explain how coherent effects give rise to a family of non-Fermi liquid states similar to the multiple channel Kondo problem. These states are remarkably stable and persist even in the resonant regime, defying conventional wisdom. As a direct consequence we predict a universal, gate-voltage independent conductance in these systems that develops at relatively high temperatures. Our results make the implementation of various devices more feasible, and we expect our observation to shift the paradigm in designing protocols for braiding Majorana modes and quantum computation away from focusing solely on the off- resonant regime.
• 15:05 15:40
Artjom Vargunin - Vortex motion and flux-flow resistivity in dirty multiband superconductors 35m FB52

### FB52

#### AlbaNova

We have developed a theoretical framework to study nonequilibrium processes in multiband superconductors based on microscopic kinetic theory and applied it to calculate flux-flow resistivity of such systems in the dirty limit with a high concentration of nonmagnetic impurities. We have considered both the regions of high and low magnetic fields. To calculate the conductivity in the former case, we have derived the solution characterizing moving vortex lattices, which reveals the effect of splitting into sublattices of fractional vortices. It is shown that in contrast to single-band superconductors, the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. This naturally explains quite diverse experimental data on the flux-flow resistivity in different multiband superconducting compounds. By choosing the ratio of diffusivities in two bands, D2/D1 = 2.5, we have obtained a quantitative agreement with experimental data for MgB2. In this case, the low-field magnetoresistance strongly exceeds the Bardeen-Stephen estimation.
• 15:40 16:10
Coffee break 30m AlbaNova entrance

### AlbaNova entrance

#### AlbaNova

• 16:10 16:45
Rogelio Dıaz- Mendez - Self-assembly and glassiness of vortex matter in type 1.5 superconductors 35m FB52

### FB52

#### AlbaNova

Type-1.5 superconductors are characterized by multiple coherence lengths, some of which are larger and some smaller than the magnetic field penetration length. This can occur in superconducting states that break multiple symmetries and in materials with multiple superconducting bands. In these conditions, vortices can display multiscale attractive and repulsive interactions leading to an exotic phenomenology both in the equilibrium and the dynamics of the vortex matter. We explore this phenomenology through numerical simulations of effective models using exact intervortex asymptotic interactions. In particular, we show that a vortex glass state can be an inherent property of a superconducting system characterized by multiple coherence lengths. Interestingly, this glassy phase emerges as a result of the self-assembly process in a clean sample with no pinning or substrate impurities.