Friday 31 July 2015- Jim Valles – Summary talk

Jim starts his talk by proposing a general scheme to classify the several topics covered by the workshop:

1-LAO/STO interfaces, status and advances

2- Pursuing related issues in superconductivity

3- SIT Physics in Thin films

4- Disorder and the SIT

5- Novel experimental probes

6- Interfaces matter

1- LAO/STO interfaces, status and advances. Jim remarks his personal surprise in discovering the beautiful world of interfaces, where the beauty actually come out from plenty of different features at play: SC, magnetism, spin-orbit (SO) scattering, gate tunability. The superconductivity is not so conventional, as shown by experiments of tunneling (Mannhart), by the Hc anisotropy (Gariglio) and the critical behavior (Bergeal). In the last case the beauty is that one can scale several samples because of the tunability in Vg, not only one or two, and this makes definitively a big advantage with respect to other system. Magnetism: it can be enhanced with a layer of ETO put in between (Stornaiuolo), giving rise to an anomalous Hall effect. One can also use Nb doped STO to boost the superconductivity (Hwang). How can we use this tunability? One proposal came out of an intrinsic spin-Hall effect induced by a modulated Rashba coupling (Seibold), motivated by a general phase-separation model (Grilli) with a possible QCP.

2- Pursuing related issues in superconductivity. Jim reviews then some other fundamental issues of superconductivity, triggered or not by the LAO/STO physics. One is the effect of strong SO coupling on superconductivity (Michaeli). The mechanisms leading to a SC dome: the one shown for granular AL (Pracht) resembles something we have seen, is it the same as in other systems or not?  Can superconductivity be triggered by a Van Hove singularity in Nanotubes (Barbara)? Can we go towards artificially-generated “molecular” superconductors (Ilani)? Superconductivity can even occurs at the nanoscale (Bose): even rare-grain effects (Mason) can be relevant to the Tc .

3- SIT Physics in thin films. Here Jim shows something exciting from himself! One can pattern Bi films with holes. According to the modulation of the underlying substrate one obtains different R(T) (direct SIT or percolation). Jim then compares flat films with hilly ones. In the latter ones he sees quantum oscillations with 2e periodicity even on the insulating side of the transition, while these are not seen in the flat films. Conclusion: if one wants a Bose insulator one needs inhomogeneity. The persistence of pairing on the I side of the SIT has been seen often during the workshop. For example, in InOx samples with very different Tc’s loose the magnetoresistance peak at the same field (Shahar), real signatures of bosons surviving across the SIT. The same occurs in decorated graphene (Bouchiat), even if here strictly speaking there is a metal in between (and plateaus..).

4- Disorder and the SIT. The route is difficult (as shown by  an explicative slide by Svortsov on Monday!). Short-scale effects can affect strongly the non-universality. So while in Leridon’s talk we have seen that the fluctuations seem to suggest OD physics (due to grains?), different disorder realizations can lead to inhomogeneous currents (Castellani), they can affect the STM DOS (Misha), and they can explain the failure of the Mattis-Bardeen picture of the conductivity near the SIT (Armitage). Here again Jim goes back to his own work in NHC films and addresses a different type of disorder, i.e. the flux disorder. How does it affect the quantum critical transport at the SIT? Jim shown that this can be realized by fabricating hole arrays with varying geometrical order. The spreading in the hole size induces magnetic-field oscillations or flux disorder (it is not ‘exactly’ one flux for hole, up to 10-20 %). Such a flux disorder increases with the magnetic field B. This implies that one can identify plateaus in the resistance at several B, like if one had multiple B-induced SIT. More remarkable, one finds out that the critical resistance as a function of flux disorder increases up to the ‘universal’ Rc, so the expectation of an universal Rc is not realized.  

5- Novel experimental probes. Here we had several nice examples: scanning critical current microscopy on a nanowire (Driessen), non-Gaussian noise in NbN (Ghosh), light-induced superconductivity in cuprates (Wanzheng Hu), a novel four-probe technique for measuring resistance in ultrathin FeSe films (Jia).

6- Interfaces matter. Jim concludes with a very nice comment: “interfaces are our friends”. They can help: one can design a Josephson junction to explore the properties of a topological superconductor (Brinkman), or one has to use a STO substrate to enhance the (tunneling) Tc of FeSe (Wang).

Overall, Jim talk showed that a common language is possible for scientists working apparently on different materials, with different experimental probes, and speaking different theoretical ‘slangs’.  And people like to discuss when there are the right conditions to do that. With Jim’s talk we close this exciting workshop and we agree that we should definitively plan to have something similar in few more years.

Blogged by Lara Benfatto

Friday 31 July 2015 - Mark Blamire - Superconducting tunneling through spin filter barriers

During this workshop, we have heard about Cooper pairs being exposed to a lot of weird conditions (as compared to the original BCS scenario) such as strong Rashba Spin Orbit coupling, extreme surface confinement, and above all, disorder of all kinds that one can think of. Then came Daniela Stornaiuolo, who proposed to make them coexist with a ferromagnetic order in new hetero-structures, by including a thin EuTiO3 layer within the regular LAO/STO interface. But the hard time for the Cooper pairs arrived when Mark forced them to cross a ferromagnetic barrier.

Indeed, Mark exposed us very beautiful experiments where Josephson and tunneling junctions are made of a thin GdN barrier sandwiched between two NbN superconducting ones. At low thickness, GdN is not magnetic, but for thickness beyond a few nanometers, a ferromagnetic behavior is observed, with a weak coercive field. In that case, the junction acts as a spin filter for 80 to 90% of the electrons. I-V characteristics shows Josephson coupling, weaker when GdN is magnetic as expected because of the strong depairing effect at play. More interesting, the Ic x Rn product (critical current times the normal state resistance) of the magnetic Josephson Junctions (JJ) deviates from the regular Ambegaokar-Baratoff law at low temperature. Finally, the Fraunhofer pattern, that is the modulation of Ic with an applied magnetic field, reveals the spin filtering effect. Indeed, it appears to be first highly asymmetric in field, and second, to shift according to the ramping direction of the magnetic field. The magnetization hysteresis loop of the GdN layer accounts for the latter behavior, since the total magnetic flux that goes through the JJ controls the critical current. However, the analysis of the asymmetry shows that, when the barrier becomes magnetic, the first order term in the current-phase relationship within the JJ is weakened, and that the second order one dominates. This is not completely understood yet, even if recent theoretical papers made interesting propositions to explain this behavior.

Then, Mark described experiments where one of the electrode of the junction is normal, in order to measure the density of states of a superconductor through a spin filtering barrier, in the spirit of the pioneer work of Tedrow and Meservey a long time ago. In that case, Zeeman splitting shifts the superconducting gap, and an offset in energy is observed in the tunneling conductance curve, corresponding to an internal field in the barrier of 1.5 T. The sign of the asymmetry tells that the magnetism acts more on one side of the junction. This offset has also been seen in JJ, indicating that there is an intrinsic asymmetry in the junctions, that could account for the disappearance of the first order term in the current-phase relation. Growth considerations and intermixing at the interfaces might explain this asymmetry.

But the story is not over, and Mark made a very nice teasing by showing us the low temperature tunneling conductance curves of the magnetic junctions, where a big zero-bias conductance peak builds up at zero energy. As Jim Valles mentioned, this immediately reminded me the old days of High Tc  superconductors, when I was discovering such an anomaly in YBCO/Pb junctions, which have been later on attributed to a zero-energy Andreev state at the (110) surface of a d-wave superconductor. This is obviously a different situation, but that may be a bound state of some sort.

Mark let the discussion open about this peak ... and the Cooper pairs again in a strange situation ...

Blogged by Jerome Lesueur

Friday 31 July 2015- Emilio Artacho - On the origin of the two-dimensional electron gas at the interface between insulating perovskites

Coming from the field of electronic structure and DFT, Emilio Artacho discusses “On the origin of

the 2D electron gas at the interface between insulating perovskites”. He introduces the “polar catastrophe” argument for LaAlO3/SrTiO3 – for an idealized heterostructure, the charged layers in LaAlO3 up against the neutral layers in SrTiO3 creates electrostatic boundary conditions which would be resolved with 0.5 electrons/2D unit cell for one interface, and 0.5 holes for the other.  Assuming idealized structures, and bulk stoichiometry, the electric field building up, as the LaAlO3 layers are stacked, drives charge transfer from the surface to the interface. DFT calculations (on superlattices) show this and correspond quite well with a simple parallel plate capacitor model.

Emilio wants to convey 2 key points in his talk:

1) The 0.5 interface charge is not only what you find in the simple ionic limit – rather it is robust to the realistic generalization (including covalency, etc.). To show this, he points out that the dipole moment/unit cell is an ill-defined concept, in that it is dependent on the choice of the origin (pointed out by Richard Martin in 70’s). A “dipole-free” unit cell can be chosen and thus projecting all the charge issues to the surface. David Vanderbilt used these ideas to make the “Berry’s phase connection” (in his case for ferroelectrics) in the 90’s, such that the boundaries can be treated much as is the case for topological insulators nowadays. Another analogy is the 1D Haldane chain, in that the relevant spin degrees of freedom are at the ends of the chain. Ultimately, the 0.5 charge is the robust consequence of being between two materials with different topological index.

2) The second point he emphasizes is that rather than purely dealing with bulk idealized stoichiometry (discussed above), another relevant degree of freedom is the stoichiometry – i.e., the 2DEG can be triggered by redox processes. The overall point is that the electrostatic boundary conditions can be resolved not just by “mobile electrons”, but also by defect chemistry. Ultimately, both processes are at play and may interplay.

Final points he makes include: not all carriers at the interface are mobile; disorder is expected; and that depending on origin, we may not necessarily have Mott-Anderson behavior, free carriers moving in a smoothly, weakly disordered potential.

Discussions include possible connections to phase separation scenarios previously discussed in the conference (Grilli et al.).

Blogged by Harold Hwang

Friday 31 July 2015 – Jianming Lu- Huge upper critical field of ionic gated MoS2

Jianming Lu reported on the latest experimental studies of superconductivity in transition metal dichalcogenides (TMDs) done in Prof. Ye’s group. The key aspect of these experiments is the use of ionic liquids to create electric double layer transistors, where a sheet of charge is

induced on the surface of the TMD by applying a gate voltage. This gating technique is extremely efficient due to the close (few nanometers) spacing between the charge and the material surface, creating an extremely large gate capacitance. As a result, high carrier densities can be induced in the TMD channel by applying just a few volts.

Prof. Ye’s group used this technique to study the effect of electrical doping on a variety of materials, including superconductors (ZrNCl), metals (Au), semiconductors and semimetals (MoS2 and graphene). This talk focused on Mo-based TMDs.

Ionic gating of MoS2, MoSe2, and MoTe2 revealed superconductivity on the electron-doping side for the first two materials. No superconductivity was reported on the hole-doping side and MoTe2 did not show any superconductivity. MoS2 and MoSe2 showed similar phase diagrams, with the dome-shaped superconducting phase starting at electron density n >  0.6X10^14/cm^2, and the critical temperature increased up to about 10 K for MoS2 and 6.5 K for MoSe2.

All these TMDs were multilayer flakes. However the speaker argued that the doping was mainly affecting the top layer, effectively decoupling it from the remaining layers and making this flake a monolayer superconductor.  The supporting evidence presented was: 1) the angular dependence of the critical magnetic field, showing a cusp for the field direction parallel to the MoS2 plane and 2) the superconducting transition showing a KT tail. This point generated questions from the audience on whether the same effects could be seen if doping and superconductivity were extended to more than one layer. In support to the single-layer argument, Jianming Lu mentioned that they had measured single layer samples and also found superconductivity with similar Tc and Tc dependence on doping. However, the single-layer work is still in progress therefore the data were not included in the presentation.

A striking result was the magnitude of the critical magnetic field in the direction parallel to the flake, larger than 80 T, much larger than the critical field measured for chemical doped bulk MoS2 and far exceeding the Pauli limit. The speaker argued that this large critical field can be explained by orthogonal protection due to Zeeman spin-orbit coupling, aligning the spins in the out-of-plane direction.

These new exciting experimental results show once again that ionic gating is a very powerful technique to uncover the rich physics of low-dimensional materials in the large carrier density regime.

Blogged by Paola Barbara

Thursday 30 July 2015- Karen Michaeli - Superconductivity in the presence of spin-orbit coupling: old dog, new trick

Some superconducting films display an increase of Tc in the parallel magnetic critical field. This strengthening of superconductivity calls for an explanation  involving both spin-orbit coupling (SOC) and magnetic field. With this motivation Karen introduces a continuous free-electron model in the presence of  a Rashba SOC. When a magnetic field B along x is turned on, the Zeeman field generically shifts the chiral bands in opposite directions along y. However, for small momenta q=2mu_0B/v_F the lower Rashba band no longer depends linearly on B and therefore identifies a circular Fermi surface centered at zero momentum, while the second chiral band is shifted by q giving  a SC order parameter Delta(r ) =Delta exp(iqr) similar to the FFLO case. Since the pairs in the first band don't depend on B, the decoherence effect of B only arises from the pairs in the second band.  Disorder has a non trivial effect on this finite-momentum SC state: at low disorder, pairs scattered in the smaller B-dependent branch of the Fermi surface stay there for long time and suffer a strong pair breaking. This leads to rapid decrease of the critical field Bc on the disorder scattering. Increasing disorder pairs in the B-dependent branch scatter more frequently in the B-independent branch and suffer less pair-breaking leading to a strengthening of SC and a recovery of the critical field with disorder. This justifies the choice of a model where disorder is assumed to  kill triplet SC, while the singlet finite-momentum pairing is mildly affected.

The SC state in the presence of B is also characterized by a finite magnetization which enters the free energy via the SOC. According to the Edelstein magnetoelectric effect, the supercurrent is accompanied by a transverse magnetic moment, which also acquires a monopole structure when a supercurrent vortex is present. 

The (Gibbs) free energy is then transformed passing to a lattice  XY model having additional terms arising from the Rashba-like magnetoelectric terms. Once the magnetic degrees of freedom are integrated out one obtains a free energy for a classical spin model with nearest-neighbor ferromagnetic coupling (favoring uniform SC) and a frustrating term proportional to the Rashba SOC leading to an helical magnetic solution corresponding to finite-momentum SC. Finally one sees that the presence of the external magnetic field enhances the superfluid density and it extends the region with helical magnetization. This indicates that the increased stability of SC under parallel B might be related to an exotic finite-momentum superconducting state.

Blogged by Marco Grilli

Thursday 30 July 2015 – Shahal Ilani - Attraction by Repulsion: Pairing electrons by electrons

Shahal Ilani addresses a highly fundamental question in a very novel and elegant approach. The question is : « Can we make 2 electrons attract only via repulsive interaction ? ».

Of course this problem is particularly relevant in the framework of superconductors and in the search for room-temperature superconductivity, since the lighter is glue, the stronger is the coupling to be expected. The approach is here to specifically design a minimal building block to obtain attraction between 2 electrons.

Starting from ideas proposed by Little where a 1D conducting molecule is coupled to a specific medium acting as an electronic polarizer and having negative dielectric constant, Ilana and his group have designed a specific system made of  a very clean carbon nanotube (CNT) containing two quantum wells and electrically connected to a measurement setup.  The electronic polarizer is also a carbon nanotube with two quantum wells. When the electronic polarizer (EP) is sufficiently far away, measurement of the voltages in the CNT shows that the electrons in the two quantum wells repulse each other, as expected, whereas when the EP is close to the CNT, the electrons are shown to attract each other i.e. there is an observable quantum state resulting of superposition of two simultaneously occupied and vacant quantum wells.  The mechanism is that when the electronic polarizer is approached to the CNT, the presence of an electron in one of the quantum wells of the CNT will provoke the hopping of the electron that is in the closest quantum well of the EP to the other (further) quantum well, therefore creating an electric field of opposite sign. In order to share the cost for the electrical field that is created, two electrons will have a tendency to occupy/disoccupy simultaneously the nearest quantum wells on the CNT.

The authors have inspected the detuning dependence of the phenomenon and are currently exploring transport in these devices.

Blogged by Brigitte Leridon

Thursday 30 July 2015 - Paola Barbara - Electronic transport and superconductivity at van Hove singularities in carbon nanotubes

Paola has worked for more than 10 years on the experimental observation of superconducting

instabilities in carbon nanotubes. She presents here some overview of the field and its challenges. The observation of intrinsic Superconducting instabilities in sp2 -hybridized carbon nanophases is a long-standing issue. Hints of superconducting transitions have been observed so far in many carbon phases but reproducibility is difficult to obtain and no clear picture of the mechanism responsible for this transition has emerged so far.

One of the most promising mechanisms to induce superconductivity is to tune the chemical potential of the nanostructure close to a Van Hove singularity, where the electron-phonon coupling is showing some singularity.

In monolayer graphene, this requires to shift the Fermi Level up to 2eV, a position impossible so far to obtain experimentally as it requires to electrostatically dope graphene above 10^15 electrons/cm2. However in twisted graphene bilayers or in carbon nanotubes, such Van Hove singularities occurs at much lower energy and are within reach of gating.

Paola then explains Carbon nanotubes (CNTs) are best candidates to observe the effect of Van Hove singularities and to tune them. There are atomically thin materials thus Paola emphasize that there properties varies a lot when one varies their size by one unit cell. For example in CNTs, the electronic properties are changing from a semiconductor to a metal with just one more carbon atom along its perimeter. bandgap is inversely proportional to the diameter of the  nanotube and put the first Van Hove singularity to a fraction of eV.

Paola is presenting a new set of experiments in which she showed that it is indeed possible to push a CNT transistor at a Van Hove singularity. This singularity is detected by the measurement of the capacitance. Indeed the quantum component of the total capacitance shows a rapid change at the Van Hove singularity. To confirm that, she has measured the quantum capacitance of the CNT at low temperature by observing the Fabry-Perot effect. At the singularity the CNT show some anomaly in the periodic oscillation indicating that a rapid change of the electronic compressibility is occurring.  At low temperature, at this precise gate voltage, she also observed a zero Bias anomaly associated by a drop of a factor 2 of resistance. This is associated to a critical temperature of about 30K.  A second device probing another portion of the same nanotube with different length is also showing a similar effect but with different parameters.  Further measurements will assess for the observation of these instabilities and will probe deeper the properties of this exotic effect.

Blogged by Vincent Bouchiat

Thursday 30 July 2015 - Eduard Driessen - STM experiments on electrically connected superconducting TiN nanowires locally driven out of equilibrium

Eduard Driessen presents a new tool to study the non-equilibrium properties of disordered superconductors he entitles „critical current microscopy“: A STM current is used to locally perturb a nanowire where the non-equilibrium state is probed by the critical current.

Eduard motivates his talk with possible applications of disordered superconductors for e.g. single photon detectors. At the same time he points at the so-far only poorly understood microwave response in strongly disordered TiN: The DOS one needs to feed into Mattis-Bardeen equations to fit the dynamical conductivity does not compare to the tunneling DOS. 

Eduard continues with the main objective of his talk: We know (maybe, a bit) about the equilibrium properties of disordered superconductors but basically nothing about the non-equilibrium properties. With the technological state-of-the-art possibilities of structuring at the nanoscale this has become feasible. The systems Eduard tailors and studies are superconducting TiN nanowires (4µm x 200nm, 1KOhm -> close to the SIT) at 50mK through which a current is sent. Using a blunt STM tip positioned above the wire, a small current (0.2 - 2nA) is locally injected. To prevent the tip crashing into the insulating substrate, the nanowire is neighbored by other nanowires separated by a distance small enough to keep the tip above the substrate.  The current through the wire is ramped up until its critical value is reached.

Prior to the non-equilibrium properties, Eduard addresses the properties with the perturbation switched off. On average, only very small coherence peaks are present and a large zero-bias anomaly (>0.5) is observed.  Eduard states that it is impossible to fit the particular shape to any formula known (either a fit captures the suppression of coherence peaks and fails at the zero-bias or vice versa). Also on the local scale, a fully gapped DOS is not resolved. This is in some sense different compared to what was observed previously (studies of B. Sacepe) and in disordered NbN, where the coherence peaks are also suppressed, but a fully gapped DOS is observed at various sites, as Pratap Raychaudhuri points out. The (equilibrium) critical current is 1.2µA.

Upon switching on the STM current injection (1nA), Eduard reports a tremendous drop of the (non-equilibrium) critical current from 1.2µA to 250nA. (“like pouring a glass of water into a river which thereupon stops flowing”). At different spots along the wire the critical current varies by 25% (qualitatively) similar to the observed variation in the (equilibrium) zero-bias map. The correlation between the variation of critical current and local DOS in the sense “low critical current <-> weak spots”, however, is delicate, as also geometric influences need to be considered. In general, weaker SC is more likely to be found close to the edges of the wire, similar as it has been inferred from optical-absorption studies.

It turns out that Ic depends on the quasiparticle energy. Starting at several meV, the critical current increases as the energy goes down, experiences a maximum before it is suppressed again. This is somewhat counterintuitive as in the limit of low quasiparticle energy the impact on the critical current becomes more severe. Eduard drops the idea of the interplay between thermalization and recombination as a possible explanation. Using a higher STM current, the absolute values of Ic are reduced, however the energy dependence persists. He notes that the maximum of the Ic curve happens to roughly coincide with the energy gap of the tunneling spectra. Something to think about.

In summary, Eduard presents a powerful new method to look at the non-equilibrium properties of disordered superconductors - apparently a field full of open questions – and a fascinating first view on its nature.  

Blogged by Uwe Pracht

Thursday 30 July 2015 – Arindam Ghosh -Probing superconductivity in low-dimensional systems with conductivity noise

We have heard a fine talk by Arindam Ghosh from the Indian Institute of Science, Bangalore, in which he presented results from noise and fluctuations measurements of thin-film NbN superconductors. The central aspect here, in my view, is that in the long history in the study of disordered superconductors (and superconductors in general) not much attention was payed to noise measurements. This is a refreshing approach, which immediately led to interesting results regarding non-Gaussian distribution in the interesting region near the BKT transition. Personally I would be very happy if the technique would be applied near the superconductor-insulator transition, and perhaps noise measurements could be used as another means to probe the nature of charge carriers in the insulation state terminating superconductivity. I am sure this is in the making. I am also aware of other groups applying noise measurements to superconducting systems and Josephson junctions with surprising results.

Blogged by Dan Shahar

Wednesday 29 July 2015 - Jinfeng Jia -Superconductivity in single-layer films of FeSe with a transition temperature above 100 K

Dr. Jia presented very interesting result on high temperature superconductivity in FeSe monolayer thin films grown on Nb doped metallic SrTiO3 grown through molecular beam epitaxy. He performed in-situ 4-probe transport measurements (critical current from I-V characteristics and temperature/magnetic field dependence of resistance) on the FeSe thin films using a specially designed spring loaded 4-probe contact. The separation between the contacts were 10 micron. He setup allowed him to perform measurements on different areas on the surface by moving his contacts around. Many points on the sample (though not all) showed a clear existence of critical current. The Tc was estimated in two different ways: The first was by plotting Ic (not all at the same location) as a function of temperature. The second way was by measuring the resistance versus temperature at a single point. Both measurements showed a very high Tc ~ 110 K, which is an order of magnitude larger than bulk FeSe. Similar measurements in magnetic field also revealed a very high upper critical field. At 96 K the measured Hc2 ~ 8 T. The estimated Hc2 from the slope of Hc2-T close to Tc, was ~115 T.

The results triggered a lot of discussion on what could be origin of such a high critical temperature. It was suggested by the speaker that doping from Nb-doped SrTiO3 could be playing a role. It was also noted that monolayer FeSe films on graphene did not show superconductivity. The other surprising aspect of the result was the  sharpness of the transition in R-T measurement. It was pointed out that for a monolayer films the transition in zero field is expected of the BKT type, and BKT fluctuations should broaden the transition. However, in this meeting similar sharp transition was also shown for interfacial superconductivity in LAO/STO interface (Mannhart). This seeems to be an outstanding puzzle and direct superfluid density measurements could shed light on this issue.

In the last part of the talk Dr.Jia presented scanning tunneling spectroscopy data on Bi2Se3 islands deposited on superconducting NbSe2 crystals. He pointed out some differences of the spacial and field dependence of the zero bias conductance peak observed inside the vortex core, in bare NbSe2 and Bi2Se3-NbSe2. He claimed that this difference was an evidence of Majorana Fermion in the vortex core. However, there is no theoretical model to associate these differences to Majorana Fermion. To this blogger these results looked very tentative at the moment.

Blogged by Pratap Raychaudhuri

Wednesday 29 July 2015 – Lili Wang -The superconductivity in single layer FeTe1-xSex films on SrTiO3

LiLi Wang started by motivating her experimental work as an effort to create higher temperature superconductors. Displaying a figure showing the discovered superconducting Tc’s over time, she pointed out that the high Tc materials exceeded expectations based on the established electron phonon interaction based models of superconductivity.  Predictions of those models were distilled by McMillan by a formula that suggested a maximum for Tc limited by the electron-phonon coupling.  LiLi noted that the high Tc materials have lower carrier densities and higher Debye temperatures than the pre-high Tc superconducting materials.Thus, materials with these two characteristics are interesting to pursue. LiLi next described two inspirations for increasing Tc:  Ginzburg predicted that superconducting states could form at the surfaces of materials.  These superconducting energy gap of these states would have to be larger than the bulk gap.  The second inspiration came from experiments showing superconductivity at the interface of two different semi-conductors by Fogel and coworkers. 

Armed with this motivation LiLi and her coworkers started searching for superconductivity in metal films on ceramic dielectric substrates with high dielectric constants.  LiLi presented STM and transport results on FeSe and FeTeSe on STO substrates.  FeSe superconducts with a bulk Tc of 9 K at ambient pressure and 37 K at high pressure.  Te substitution can raise the bulk Tc to 15K at ambient pressure. 

LiLi presented a comparison of FeSe films grown on STO and grown on graphene to understand how the substrate modified the FeSe film properties.  On graphene, FeSe forms islands that are weakly mechanically coupled to the graphene.  The square lattice spacing a was 0.38 nm, which is close to the bulk value.  On STO, the FeSe forms a nearly uniform epitaxial film with a=0.39 nm, which is the STO lattice constant. 

 The FeSe films on graphene show a BCS like quasiparticle tunneling characteristic as measured by in situ STM.  Unlike BCS, however, the sub gap conductance has a linear energy dependence.  The superconducting Tc as determined from the temperature dependence of the zero bias conductance decreased as 1/d where d is the FeSe film thickness to disappear at an extrapolated thickness of dc=0.7 nm. 

By contrast, FeSe films on STO only showed a superconducting quasiparticle tunneling characteristic when they were 1 UC thick.  Thicker films showed no sign of superconductivity.  Remarkably, the conductance showed a large gap with vertical edges, two peaks at positive and negative polarities and a very low zero bias conductance.  The presence of two peaks suggests that there are two energy gaps in these films with 2\Delta=25 and 40 meV.  The gap closes when the temperature is raised up to 68 K implying a substantially enhanced Tc in these FeSe films.  ARPES data from another group also exhibits a large gap (\Delta=15-19 meV) that forms on the surface of 4 electron pockets centered on the M points in the Brillouin zone.  The gap looks isotropic. 

To corroborate the high Tc values, LiLi developed a method to cap the FeSe for ex situ transport measurements.  Films capped with a 10UC FeTe and 30 nm thick amorphous Si film exhibited a resistively measured midpoint Tc of 32 K and a inductively measured Tc of 21 K.  Both of these values are lower than the in situ STM measurements reveal.  

Next, LiLi presented STM measurements on FeTe1-x films to see how substitution affected the properties.  The gap in the tunneling conductance varied negligibly with Te additions up to 90% substitution.  100% substitution quenched the gap structure. 

At the end of her talk, LiLi rhetorically asked what may make Tc so high in these 1 UC FeSe films.  Is it a charge doping effect? This seems possible because gating can affect Tc and recent experiments by a Japanese group showed that K atoms on the surface of FeSe films could enhance their superconducting properties.  Is it a surface enhanced electron phonon coupling effect?  If so, then adding an STO layer atop the FeSe films might enhance the Tc further.  LiLi suggested that a STO/FeSe/STO sandwich may be a delectable treat for science!

Blogged by Jim Valles

Wednesday 29 July 2015 – Wanzheng Hu - Light control of correlated electron systems

Wanzheng Hu reported on some recent developments in the Hamburg group’s continuing work on the light control of correlated electron systems. 

Wanzheng started by reviewing the characteristic energy scales of solids and pointed out that conventionally performed optical pump-probe experiments typically use pump energies that far exceed many of the most interesting energy scales in solids.   The Hamburg group has taken advantage of recent technical developments to perform pump-probe experiments where the pumping is done at energies that are resonant to important excitation scales.

She briefly reviewed recent work from this group on light enhanced superconductivity on LaEuSrCuO4 and YBCO and the most recent work on K3C60.   In all these cases the claim is that by exciting the system one can stabilize a transient superconducting state at temperatures well in excess of the equilibrium Tc.

She then went on to discuss in detail her work on light induced transient superconductivity up to room temperature in YBCO.   In this work they resonantly pump the system at a frequency near an apical oxygen phonon and observe a characteristic 1/omega dependence in the imaginary part of the conductivity.   They believe that when they excite the system there are transient distortions which serve to redistribute Jospheson coupling energies from between the YBCO bilayers to between the unit cell couplings.  In this process they estimate that approximately 20% of the volume of the sample undergoes this redistribution.  Time resolved XRD has been done and is consistent with a lattice rearrangement in the transient state that is consistent with the inferred changes in the Josephson energies.

Wanzheng then shifted topics to discuss their groups work on whether or not one can excite a IR optical mode and drive a metal-insulator transition in NdNiO3 films on LaAlO3.   In this work they pump a substrate phonon and trigger a metal-insulator transition.   They observe that the THz conductivity changes by 7 orders of magnitude.   They have seen roughly similar physics in SmNiO3 films on LaAlO3.

Blogged by Peter Armitage

Wednesday 29 July 2015 - Brigitte Leridon - Confinement of superconducting fluctuations due to emergent electronic inhomogeneities in ultrathin NbN

Brigitte starts her presentation by presenting the general motivation of the presented work: understanding the effect of enhanced inhomogeneity on the superconducting (SC) properties of thin NbN films.

The principal probes are transport measurement (with or without magnetic field), TEM and STS.

First she makes the distinction between granular and homogeneously disordered systems. Granular materials are coherent with Coupled Josephson-junction. The disappearance of SC properties into granular films follows a phase fluctuation model (described by the XY model). For inhomogeneous materials, there is two major models for the SC : it’s either fermionic (localized fermion into the insolator) or bosonic (localized cooper pairs)

About the experiment: The films are grown on Al2O3 substrate and then measured ex-situ. Tc are from 9 to 2.4K and is evolving other the time, as the sheet resistance.

TEM measurement: have shown a crystallization of the sample next to the substrate on few layers and a passivation layer probably made of Nb2O5.

STS measurement: have shown gap inhomogeneity. Cross correlation map between the spectroscopic and topographic pictures show nothing. Auto-Correlation of the spectroscopic picture show a typical range of inhomogeneity distribution of  100nm. This distribution is stable between 300mK and 4K

Electronic-transport measurements have shown a non-monotonous behavior of Tc as a function of thickness . It is not following finkelstein’s theory.

By fitting the influence of fluctuations, a simple 2d Aslamasov-Larkin fluctuation model (AL) is enough to explain SC fluctuation for the cleanest samples. It seems in accordance with an amplitude fluctuation model for the destruction of SC, supposed to be in accordance with a fermionic model of the suppression of SC. There is no need to take into account Maki-Thomson or DOS correction. For more disordered samples, fluctuations fit with a 0d-AL fluctuations. There is a 0d-2d crossover near to Tc (defined as R=0). These 0d-AL fluctuation supports the picture of coupled grains which size is corresponding to the (electronic) inhomogeneity distribution.

She finished the talk by a renormalization analysis coherent with nu=2/3 and z=1, obtained from electric-field and magnetic field variation. These values are compatible with a  XY model.

To summarize, Brigitte concluded on the presence of both bosonic and fermionic features into the sample.

The question of 0d fluctuations is open, as the competition between a fermionic and bosonic picture of the destruction of the SC in disordered thin films, is open.

Blogged by Vincent Humbert

Wednesday 29 July 2015 - Nicolas Bergeal - Quantum phase transitions at oxide interface

Nicolas brings back the discussion to the superconducting oxide interfaces, talking about 

LaXO3/SrTiO3 heterostructures, where X stands for Ti or Al. Despite differences in the constituents of these interfaces (a Mott insulator facing a band insulator in the LaTiO3/SrTiO3 case vs two band insulators for the LaAlO3/SrTiO3 configuration), he announces that the physics of the 2D electron gas (2DEG), that sits in the STO side, so far, is the same.

After a short review of the relevant properties of these polar/non-polar interfaces, he starts discussing the magnetic field-driven superconductor to insulator transitions. His scaling analysis reveals two critical behaviors with two quantum critical fields: for a high temperature region (between 100 and 200 mK), the critical exponents are znu=⅔; for lower temperatures (down to 40 mK), they are equal to 3/2. This behavior, observed for positive gate voltages, persists also for negative gates, but in the latter case only one critical field is observed. These data (see publication in Nat. Mat. 2013) raise different questions in the audience: why the resistance changes just by few percent at the critical point?  Is the low T region originating from cooling issues that renormalize the T scale?

Some of the answers can be found in the model that he introduces to explain these results: it is based on dilute superconducting puddles coupled by the 2DEG, discussed yesterday in detail by Marco Grilli.

When the superconductor to insulator transition is induced by sweeping the gate voltage, the scaling analysis reveals critical exponents equal to 1.6, close to 3/2. This value, despite having being already observed for instance in high temperature superconductors, it is difficult to justify theoretically with existing models. The new scenario they consider is based on density driven fluctuations, as discussed by Marco. The picture is the one of a material where a phase separation generates regions with high electron density and low electron density and their interplay determines the physics. This scenario asks confirmation from local spectroscopy.

Blogged by Stefano Gariglio

Wednesday 29 July 2015 – Vincent Bouchiat - Field-effect controlled superconductivity in metal-decorated graphene

The seminar by Vincent Bouchiat on “Field effect controlled superconductivity in metal decorated graphene” consisted a systematic study of proximity-induced superconductivity in graphene created with either randomly distributed or lithographically patterned arrays of tin nanoparticles/discs. The underlying idea of the work is very interesting because it exploits the field effect properties of graphene, as well as the tunability of disorder, to explore several contemporary questions and issues in purely two dimensional superconductivity, ranging from the nature quantum phase transition from the superconducting to the metallic (or the insulating) state to granular superconductivity and emergent intermediate metallic state.

The talk was laid out nicely with appropriate chronological and conceptual sequence. Dr. Bouchiat first described the work on graphene decorated with dense randomly distributed Sn adsorbates. The 2D nature of proximity-induced superconductivity in graphene seem to favor phase fluctuations and a gate tunable Berezinskii-Kosterlitz-Thouless (BKT)-type scaling of the temperature-dependence of resistance across the transition. It was then shown that similar experimental platform can be used to achieve a superconductor-insulator transition too, by using deliberately damaged CVD graphene (via chemical means) as the host material. It was interesting to see that the transition occurred at the universal magnitude of h/4e², expected from the self-dual Bosonic scenario, although the universality of this observation was not clear to me. Another interesting aspect was the exponent obtained from the finite size scaling, which did not correspond to the classical percolation picture. The speaker speculated if this could correspond to quantum percolation processes.

The final experimental aspect was superconductivity induced in graphene by a dilute but regular array of the Sn dots. The most interesting observation here is the anomalous reduction of the sample's critical temperature T_c with respect to the calculated T_BKT on increasing graphene resistance, leading to a sudden collapse of superconductivity when approaching the charge neutrality point of the graphene layer. Dr. Bouchiat believes this deviation is due to the breakdown of the superconducting phase stiffness due to the emergence of quantum phase fluctuations, rather than thermal phase fluctuations which is addressed in conventional BKT-type framework.

In summary, I wish to note that the use of graphene as the experimental platform seems to be motivated mainly by the 2D nature of graphene rather than its unique band structure or Dirac Fermionic quasiparticle excitations. For example, Cooper pairs in graphene’s hexagonal lattice are expected to give rise to nontrivial Andreev reflection, which were not encountered in these experiments. The experiments presented in the seminar not only demonstrate very similar features observed and debated in other classes of low-dimensional superconductors such as the oxide interfaces or ultra-thin NbN films, but also emphasize the applicability of graphene as a versatile system with which many of the outstanding questions can be investigated with great control. 

Blogged by Arindam Ghosh

Wednesday 29 July 2015- Nadia Mason - Engineering Interactions in Arrays of Superconducting Islands

Superconductivity in artificially engineered, completely tunable arrays of superconducting islands has been a dream for experimentalists working in the area of mesoscopic superconductivity. Nadya’s talk focussed on her work in such tunable systems grown in her lab. She started by explaining how electron beam lithography is used to pattern films consisting of arrays of Nb islands of typical size of 90-150 nm on a 10 nm thick Au film which has underlying leads to carry transport measurements. Both the island size which is much larger than the superconducting coherence length (~ 27 nm) as well as the spacing between them which is larger than the normal metal coherence length can be tuned (~270/√T, T being the temperature). The Nb islands itself consisted of small Nb grains (~ 2-3 nm diameter) grown by e-beam evaporation. From the transport measurements they could observe two transitions. The transition at higher temperature (T1) corresponded to the individual Nb islands becoming superconducting while the transition at lower temperature (T2) related to the temperature at which the islands become Josephson coupled to give global superconductivity. T2 decreased with increasing spacing between the islands confirming this scenario. However, interestingly T1 also decreased with increased island spacing which formed the motivation for the second part of her talk.

Nadya also briefly mentioned that for very dilute systems with large island spacing, they could see the transition to the low temperature metallic state. 

Nadya also showed some of her very recent experiments on individual Nb islands grown on lithographically patterned substrates with Au contact pads. One of the intriguing results of these experiments was the observation of a strong suppression (~ almost 4 times) in Tc for individual Nb islands with relatively large sizes (~200-1000 nm). Nadya argued at these length scales neither of the usual culprits of proximity effect, finite size effects or charging effects could lead to the observed suppression of Tc. Moreover, different measurements on same sized islands showed larger Tc fluctuations in small islands as compared to larger ones. According to Nadya this was due to the large grain size distribution in these islands observed from transmission electron microscopy images. A simple calculation taking an exponential size distribution could explain the suppression in Tc of the micro-islands. Nadya termed this as the “rare-grain effect” where the onset of the transition is influenced by the largest grain in the island, belonging to the exponential tail of the grain size distribution. 

Nadya’s experiments opens up several questions, one of them being; How will Tc get influenced if two islands with same size showing the “rare grain effect” are brought in proximity to each other and will it be able to explain the decrease in T1 observed in the arrays with island spacing? This leaves room for further exploration in these superconducting islands and tunable arrays.

Blogged by Sangita Bose

Tuesday 28 July 2015 – Goetz Seibold - Intrinsic spin Hall effect in systems with striped spin-orbit coupling

After introducing the spin Hall effect, Götz described the basics in terms of the skew scattering mechanism and the side jump, and how in the stationary state, in a homogeneous system, the intrinsic conduction is cancelled by a diffusive contribution countering it.

Against this background he proposed to look at spatially non-homogeneous systems, and proposed a periodic modulation of the Rashba spin-orbit coupling for the 2-dimensional electron liquid (2DEL) at the interface between a thin lanthanum aluminate film and a strontium titanate substrate (LAO/STO). The modulation proposed could be achieved, e.g., putting periodically spaced metallic stripes as top electrodes to dope in a one-dimensionally

varying manner. The spin-orbit  coupling would be thus modulated, and the carrier concentration would also oscillate with maximal density for maximal coupling. The theory  is, however, much more general, quite applicable to very different systems, while keeping the periodic stripe geometry on a 2D system.

In this scenario, it seems that even the ground-state would display non-zero spin currents. Considering now the system in the presence of an applied electric field, but in the stationary situation, one finds suitable conditions for which one can get a non-zero spin-hall sigma, while keeping a zero “stationarity parameter” gamma  (i.e. the one which identifies the spin-hall conductivity of the equivalent homogeneous case).

Such a behaviour would be related to localised states at the stripes in the 2DEL. This Rashba spin-orbit coupling characteristics should be robust against disorder.

The charge transport would be strongly suppressed in the direction of the applied field, giving rise to large spin-hall angles.

The presentation was on the technical side, but offering good insights into the physics of the problem. As normal with last lectures in a dense session that is running late, the discussion was brief.  There was a question on the range of density where this effect is robust. The answer was that it corresponds (in the homogeneous- Rashba-coupling case) to the regime where a single chiral band is occupied. However, for the inhomogenenous case such an identification is not so simple. A second question was on the possibility to achieve the same effect with different inhomogeneous profiles for the Rahba coupling, and the answer was that in a previous work it was shown that if the Rashba coupling oscillates randomly aorund a fixed value it cannot be seen.

Blogged by Emilio Artacho

Tuesday 28 July 2015 - Stefano Gariglio - Study of Superconductivity at LaAlO3/SrTiO3 Interfaces by Field Effect

Stefano starts his talk by a general introduction on the fascinating properties of the LaAlO3/SrTiO3 (LAO/STO) interface, namely superconductivity, strong spin-orbit coupling and magnetism. In this interface between a non-polar material (STO) and a polar material (LAO), an electronic reconstruction takes place to avoid the divergence of the electrostatic potential as the LAO thickness increases. As a result, electrons are transferred from the surface of the LAO to the STO where Ti ions can have a mixed-valence ionic state. A 2DEG is formed at the interface and extends in the STO on a typical thickness of 10 nm. We will get back to this point latter. One key question that Stefano addressed is whether or not this electronic reconstruction is specific to the LAO/STO interface. The answer is...no and several other oxides interfaces have been found to be conducting and even superconducting with similar transition temperatures than the one of LAO/STO. But of course, one always needs the same ingredient : a polar discontinuity. Let's look at a few examples. If you replace the LaAlO3 layer by a LaGaO3 layer, another wide gap and polar material, you also obtain a 2DEG which is superconducting below 300mK. Not very surprising since Ga has the same valence state than Al (just one row below in the periodic table). A 2DEG is also observed with LaTiO3 and LaVO3, two Mott insulators where the transition metal ion (Ti or V) can take many different valence states. The LaTiO3/SrTiO3 interface is also superconducting but so far there is no report of superconductivity in the LaVO3/SrTiO3 one. Good, but these are all STO based heterostructures.  Can we use another non-polar material to replace STO ? The answer is yes. H. Hwang and collaborators have shown that under proper atomic boundary conditions, the TiO2/LaAlO3 interface is conducting with a high-mobility.

Then Stefano reported a measurement of both the perpendicular and the parallel critical magnetic field of superconducting LAO/STO interfaces over the entire phase diagram (i.e. as a function of electrostatic back gating). By checking carefully the temperature dependence of these two critical fields, it is in principle possible to discriminate between single-gap and two-gap superconductivity. That's an important point for the LAO/STO interface since, back in the early days, two gaps have been measured in bulk doped STO by tunneling spectroscopy. As far as we can say from the experimental data of Stefano, it seems that there is only one superconducting gap in the LAO/STO interface.

Now let's go back to the extension of the 2DEG in the STO substrate. From the perpendicular critical magnetic field, Stefano extracted the GL coherence length has a function of  back gate voltage or equivalently as a function of the sheet conductance.  As expected, the coherence length takes a minimum value when the Tc is maximum (for a conductance of approximately 1 mS) and follows an inversed dome shape. From the parallel critical field, Stefano extracted the thickness of the superconducting 2DEG using again a simple GL picture. For low conductance (negative gate voltage), the extension of the 2DEG in the STO is about 10 nm and it increases up to 30 nm for the highest conductance (positive gate voltage). This result is consistent with the electrostatic filling of the highest subbands in the interfacial quantum well, which delocalize deeper in the STO substrate. This is a beautiful result, which should help us to better understand the superconducting properties of these interfaces as a function of electrostatic gating. However, it also raises a very fundamental question. This analysis being entirely based on a simple GL picture, how can we explain that the parallel critical field exceeds by far the Pauli limit? Everybody agrees that the strong Rashba spin-orbit coupling probably plays a role into this but this key point still needs to be clarified.

Blogged by Nicolas Bergeal