Giovanni Ummarino

Politecnico di Torino | polito · DISAT - Department of Applied Science and Technology

We calculated the critical temperature in the framework of s±+d-wave multiband Eliashberg theory. We have solved these equations numerically to see at what values of the input parameters there is a solution with a non-zero critical temperature and what is the symmetry of the order parameter of this solution. For our model we consider the pnictide c...

We review experiments and theoretical models about the possible mutual interplay between the gravitational field and materials in the superconducting state or other macroscopic quantum states. More generally, we focus on the possibility for quantum macrosystems in a coherent state to produce local alterations of the gravitational field in which the...

We estimate the conjectured interaction between the Earth gravitational field and a superconductor immersed in external, static electric and magnetic field. The latter is close to the sample upper critical field and generates the presence of a vortex lattice. The proposed interaction could lead to multiple, measurable effects. First of all, a local...

We estimate the conjectured interaction between the Earth’s gravitational field and a superconductor immersed in external, static electric and magnetic field. The latter is close to the sample upper critical field and generates the presence of a vortex lattice. The proposed interaction could lead to multiple, measurable effects. First of all, a loc...

Experimental data from the recently discovered iron-based superconductor (IBS) KCa2Fe4As4F2 is analyzed within a realistic five-band s± Eliashberg model with coupling provided by antiferromagnetic spin fluctuations. Fundamental parameters are deduced from available angle resolved photoemission spectoscopy (ARPES) and ab-initio density functional th...

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+δ(BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that...

We calculate the possible interaction between a superconductor and the static Earth’s gravitational fields, making use of the gravito-Maxwell formalism combined with the time-dependent Ginzburg–Landau theory. We try to estimate which are the most favorable conditions to enhance the effect, optimizing the superconductor parameters characterizing the...

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg's theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+d (BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate th...

We calculate the possible interaction between a superconductor and the static Earth's gravitational fields, making use of the gravito-Maxwell formalism combined with the time-dependent Ginzburg-Landau theory. We try to estimate which are the most favourable conditions to enhance the effect, optimizing the superconductor parameters characterizing th...

We perform single- and multi-band Migdal-Eliashberg (ME) calculations with parameters exctracted from density functional theory (DFT) simulations to study superconductivity in the electric-field-induced 2-dimensional hole gas at the hydrogenated (111) diamond surface. We show that according to the Eliashberg theory it is possible to induce a high-T...

Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ thin superconducting film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow ($D_1$) and another broad one ($D_2$). In the superconducting state, two gaps with $2\Delta _{...

We perform electric double-layer gating experiments on thin films of niobium nitride. Thanks to a cross-linked polymer electrolyte system of improved efficiency, we induce surface charge densities as high as $\approx 2.8 \times 10^{15}\,\mathrm{cm}^{-2}$ in the active channel of the devices. We report a reversible modulation of the superconducting...

In this paper we examine the possibility of a Josephson AC effect between two superconductors induced by the Earth’s gravitational field, making use of the gravito-Maxwell formalism. The theoretical framework exploits the symmetry between the weak field expansion of the gravitational field and the standard Maxwell formulation, combined with the Jos...

I solved the Eliashberg equations for a multiband non-phononic s± wave spin-glass superconductor, calculating the temperature dependence of the gaps and of superfluid density. Their behaviors were revealed to be unusual: showing non-monotonic temperature dependence and reentrant superconductivity. By considering particular input parameters values t...

Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe1.91Ni0.09As2thin superconducting film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow (D1) and another broad one (D2). In the superconducting state, two gaps with 2Δ(2)0/kBTc= 1.9-2.0 and 2Δ(1)0/k...

Underdoped PrFeAs(O,F), one of the lesser known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, magnetometry, nuclear magnetic resonance (NMR) measurements, and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFe...

In this paper we examine the possibility of a Josephson AC effect between two superconductors induced by the Earth's gravitational field, making use of the gravito-Maxwell formalism. The theoretical framework exploits the symmetry between the weak field expansion of the gravitational field and the standard Maxwell formulation, combined with the Jos...

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of P...

We report on the Eliashberg analysis of the electrodynamic response of Ba(Fe1−xRhx)2As2 single crystals across the disorder-induced s± to s++ transition. We previously experimentally identified the transition by its signature in the low temperature value of the penetration depth and observed a peculiar dependency of the critical temperature on diso...

We report on the anisotropy of the London penetration depth of CaKFe4As4, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by Ni substitution) and irradiation induced (by 3.5-MeV protons). Indeed, CaKFe4As4 is particularly suitable for the study of fundamental superco...

I solved the Eliashberg equations for multiband non-phononic $s\pm$ wave spin glass superconductor and I calculated the temperature dependence of the gaps and superfluid density that reveals unusual behavior as non monotonic temperature dependence and reentrant superconductivity. For particular values of input parameters the phase diagram is still...

The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from terahertz spectra of conductivity and dielectric permittivity of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film with critical temperature $T_c=19.6$~K. Part of experimental data were analyzed within a simple three-band Eliashberg model w...

The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from the terahertz spectra of conductivity and the dielectric permittivity of BaFe 1.91 Ni 0.09 As 2 film with critical temperature T c = 19.6 K. Part of the experimental data were analyzed within a simple three-band Eliashberg model...

We perform single- and multi-band Migdal-Eliashberg (ME) calculations with parameters exctracted from density functional theory (DFT) simulations to study superconductivity in the electric-field-induced 2-dimensional hole gas at the hydrogenated (111) diamond surface. We show that according to the Eliashberg theory it is possible to induce a high-T...

In this work we combine ab‐initio density functional theory computations and the self‐consistent solution of one‐band s‐wave generalized Eliashberg equations with proximity effect in order to explain 60 years old experimental data on how a static electric field can affect the superconductive critical temperature of Indium thin films. While electron...

We study the behaviour of a superconductor in a weak static gravitational field for temperatures slightly greater than its transition temperature (fluctuation regime). Making use of the time-dependent Ginzburg–Landau equations, we find a possible short time alteration of the static gravitational field in the vicinity of the superconductor, providin...

Recent experimental data (H. Nam et al., Phys. Rev. B 100, 094512 (2019)) of critical temperature and gaps measured on superconductor/normal metal heterostructure (Pb/Ag), epitaxially grown, shown a interesting not usual behaviour. The critical temperature decreases strongly but, despite the large differences in the lattice constants and electronic...

Recent experimental data (H. Nam et al., Phys. Rev. B 100, 094512 (2019)) of critical temperature and gaps measured on superconductor/normal metal heterostructure Pb/Ag, epitaxially grown, shown a interesting not usual behaviour. The critical temperature decreases strongly but, despite the large differences in the lattice constants and electronic d...

We study the behaviour of a superconductor in a weak static gravitational field for temperatures slightly greater than its transition temperature (fluctuation regime). Making use of the time-dependent Ginzburg-Landau equations, we find a possible short time alteration of the static gravitational field in the vicinity of the superconductor, providin...

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ (CaK1144) for $x=$0, 0.017 and 0.034. To obtain the superfluid density, $\rho=\left(1+\Delta \lambda_L(T)/\lambda_L(0) \right)^{-2}$, the temperature dependence of the London penetration depth, $\Delta \lambda_L (T)$, was measured by usi...

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe1−xNix)4As4 (CaK1144) for x=0, 0.017, and 0.034. To obtain the superfluid density ρ=1+ΔλL(T)/λL(0)−2, the temperature dependence of the London penetration depth ΔλL(T) was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave...

We calculate the effect of a static electric field on the superconductive critical temperature of Indium thin films in the framework of proximity effect Eliashberg theory, in order to explain 60 years old experimental data. Since in the theoretical model we employ all quantities of interest can be computed ab-initio (i.e. electronic densities of st...

We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of BaFe2As2 single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to th...

A key aspect of field effect experiments is the possibility to induce charges on the first layers of a sample as a function of an applied gate voltage. It is therefore possible to study correlated phases of matter as a function of the induced charge density and the applied electric field. Moreover, resulting charge modulation along the direction of...

Microwave measurements of the London penetration depth and critical temperature Tc were used to show evidence of a disordered-driven transition from s± to s++ order parameter symmetry in optimally doped Ba(Fe1−xRhx)2As2 single crystals, where disorder was induced by means of 3.5 MeV proton irradiation. Signatures of such a transition, as theoretica...

In a recent paper, we presented the first directional point-contact Andreev reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure and discussed the pressure dependence of the gaps and the critical temperature. While Tc exhibits a well-known smooth dependence and a broad maximum at about 0.6 GPa, both gap...

We report on direct measurements of the energy gaps (carried out by means of point-contact Andreev reflection spectroscopy, PCARS) and of the critical temperature in thin, optimally doped, epitaxial films of BaFe2(As1-xPx)2 irradiated with 250 MeV Au ions. The low-temperature PCARS spectra (taken with the current flowing along the c axis) can be fi...

We investigate the effects of pure charge doping (obtained by using the so-called electric-double layer gating technique) in thin films of the metallic superconductor NbN. Despite the electrostatic screening due to the large intrinsic carrier density of this material, we observe measurable, ambipolar modulations of the superconducting critical temp...

One of the open issues concerning iron-based superconductors is whether the s± wave model is able to account for the overall effects of impurity scattering, including the low rate of decrease of the critical temperature with the impurity concentration. Here we investigate Ba1−xKxFe2As2 crystals where disorder is introduced by Au-ion irradiation. Cr...

We calculate the effect of a static electric field on the critical temperature of a s-wave one band superconductor in the framework of proximity effect Eliashberg theory. In the weak electrostatic field limit the theory has no free parameters while, in general, the only free parameter is the thickness of the surface layer where the electric field a...

We provide the detailed calculation of a general form for Maxwell and London equations that takes into account gravitational corrections in linear approximation. We determine the possible alteration of a static gravitational field in a superconductor making use of the time-dependent Ginzburg–Landau equations, providing also an analytic solution in...

We investigate the penetration depth of high-quality Ba1−xKxFe2As2 single crystals by a planar waveguide resonator technique, in a cavity perturbation approach. The experimental λL is compared to calculations based on the three-band Eliashberg equations within the s± wave model. To this end, the anisotropy of the penetration depth is taken into acc...

The electrochemical gating technique is a powerful tool to tune the surface electronic conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows obs...

Recent angle-resolved photoemission spectroscopy measurements of CaKFe4As4 report the presence of four superconductive gaps on different sheets of Fermi surface. The interesting aspect of this superconductor is that it is stoichiometric and with a high critical temperature. I show that the phenomenology of this superconductor can be explained in th...

We perform electric double-layer gating experiments on thin films of niobium nitride. Thanks to a cross-linked polymer electrolyte system of improved efficiency, we induce surface charge densities as high as ≈ 2.8 × 1015cm−2 in the active channel of the devices. We report a reversible modulation of the superconducting transition temperature (either...

We show that the phenomenology of the iron chalcogenide superconductor FeTe1−x Sex can be explained within an effective three-band -wave Eliashberg model. In particular, various experimental data reported in literature—the critical temperature, the energy gaps, the upper critical field, the superfluid density—can be reproduced by this model in a mo...

The temperature dependence of the resistivity of epitaxial Ba(FeCo)As thin films (with nominal doping and ) has been analysed and compared with analogous measurements on single crystals taken from literature. The of thin films looks different from that of single crystals, even when the cobalt content is the same. All curves can be fitted by conside...

We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders...

Iron-based superconductors (IBS) display a surprising variety of properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact A...

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting s...

The temperature dependence of resistivity $\rho(T)$ of epitaxial Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin films (with nominal doping x=0.08, 0.1 and 0.15) has been analyzed and compared with analogous measurements on single crystals taken from literature. The $\rho(T)$ of thin films are different from that of single crystals, although they both have the...

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1−x}Co_{x})_{2}As_{2} thin films on the nominal doping x (0.04⩽x⩽0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps—...

The phenomenology of LiFeAs superconductor can be explained in the framework of a four-band s$\pm$-wave Eliashberg theory. We have examined the experimental data available in literature and we have found out that it is possible to reproduce the experimental critical temperature, the gap values and the upper critical magnetic field within an effecti...

We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev reflection spectroscopy. The results show that the SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} system keeps a multigap character at least up to x=0.50, and that the gap amplitudes Delta_1 and Delta_2 scale...

In this paper we propose a model to reproduce superconductive and normal properties of the iron pnictide LiFeAs in the framework of the four-band spm wave Eliashberg theory. A confirmation of the multiband nature of the system rises from the experimental measurements of the superconductive gaps and resistivity as function of temperature. We found t...

We performed point-contact Andreev-reflection measurements in Ba(Fe1−x Cox )2As2 single crystals (x=0.1, T c =24.5 K) and SmFeAsO1−x Fx polycrystals (x=0.2, T c =52 K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron–boson interacti...

The experimental data of four iron-pnictides superconductors: LaFeAsO1−x Fx , Ba1−x Kx Fe2As2, SmFeAsO1−x Fx , and Ba(Fex Co1−x )2As2 have been examined and explained in the framework of a three bands s± wave Eliashberg theory with only two free parameters plus a feedback effect, i.e., the effect of the condensate on the antiferromagnetic spin fluc...

Superconductivity is due to an attractive interaction between electrons that, below a critical temperature, drives them to form Cooper pairs and to condense into a ground state separated by an energy gap from the unpaired states. In the simplest cases, the pairing is mediated by lattice vibrations and the wavefunction of the pairs is isotropic. Les...

Supplementary Figures S1-S3, Supplementary Tables S1 and S2, Supplementary Methods and Supplementary References

By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n(2D) as high as 3.5×10(15)  e/cm(2) in gold films and to observe large relative variations in the film resistance, ΔR/R', up to 10% at low temperature. ΔR/R' is a linear function of n(2D)-as expected withi...

In this work we investigate some effects of the breakdown of Migdal's theorem in high and low-Tc superconductors. In the cuprates the electronic bandwidth has a very low value compared to the characteristic phonon energy and, in order to reproduce the experimental Tc and tunneling data in the framework of the Eliashberg theory for strong electron-b...

In a recent paper [J. H. Schön, Ch. Kloc, R. C. Haddon and B. Batlogg, Nature408, 549 (2000)] C60 doped with holes by application of an intense electric field was found to superconduct at remarkably high temperature. In this paper we show that the observed dependence of Tc on doping can be reproduced by solving the (four) s-wave Eliashberg equation...

We present a series of results concerning the determination of the gap(s) in MgB2 by means of break-junction and point-contact measurements. We were able to observe the small 3D gap predicted by the two-band models for MgB2 in the clean limit as well as the gap in dirty limit. We were also able to follow the temperature evolution of the latter, fro...

In the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-Tc superconductors, for different possible symmetries of the order parameter (s, d, s + id, s + d, anisotropics and extendeds). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that...

We performed point-contact experiments on high-quality La2-xSrxCuO4 polycrystalline samples with 0.116 ≤ x ≤ 0.186 by using very sharp Au tips (~ 2μm of diameter). In most cases we observed conductance curves indicating the occurrence of Andreev reflection. The stability of the contact allowed us to study the temperature dependence of these curves...

A new possible indirect way of testing pair symmetry in high-Tc superconductors has been set up. The degree of intrinsic gap depression at Superconductor-Insulator [S-I] interfaces required to match Ic(T)Rn(T) data in HTS Josephson junctions depends on the pair symmetry of the material itself, so that an higher fraction of d-wave symmetry for the o...

We measured the ab-plane resistivity of La2-xSrxCuO4 underdoped single crystals (x = 0.052 and x=0.06) between 4.2 and 300 K by using an AC version of the Van der Pauw technique. We suggest that a possible scenario of charge-stripe ordering at the lowering of the temperature can be delineated by starting from the main features of the ρab (T) curves...

A century on from its discovery, a complete fundamental understanding of superconductivity is still missing. Considerable research efforts are currently devoted to elucidating mechanisms by which pairs of electrons can bind together through the mediation of a boson field different than the one associated to the vibrations of a crystal lattice. PuCo...

In the present letter we discuss the effect of the intrinsic surface depression of the order parameter on the temperature dependence of the Josephson critical current in high-Tc SIS and SIS′ junctions. The theoretical results are compared to the Ic(T)RN(T) curves we recently obtained from reproducible data of break junctions made at 4.2 K in single...

Point-contact Andreev reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have con...

We study the graphite intercalated compound CaC$_6$ by means of Eliashberg theory, focusing on the anisotropy properties. An analysis of the electron-phonon coupling is performed, and we define a minimal 6-band anisotropy structure. Comparing with Superconducting Density Functional Theory (SCDFT) the condition under which Eliashberg theory is able...

The phenomenology of iron pnictide superconductors can be explained in the framework of a three-band s± wave Eliashberg theory with only two free parameters plus a feedback effect, i.e., the effect of the condensate on the antiferromagnetic spin fluctuations responsible for the superconductivity in these compounds. I have examined the experimental...

The phenomenology of iron-pnictides superconductors can be explained in the framework of a three bands s\pm wave Eliashberg theory with only two free parameters plus a feedback effect i.e. the effect of the condensate on the antiferromagnetic spin fluctuactions responsible of the superconductivity in these compounds. I have examined the experimenta...

The experimental critical temperatures and the gap values of the superconducting materials LaFeAsO1−x Fx , SmFeAsO1−x Fx and Ba1−x Kx Fe2As2, which are exponent of the most important families (1111 and 122) of iron pnictides, can be reproduced by an effective s-wave three-band Eliashberg model in which the dominant coupling is in the interband chan...

Directional point-contact Andreev-reflection measurements in Ba(Fe(1-x)Co(x))2As2 single crystals (T(c) = 24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The point-contact Andreev-reflection spectra also feature additional structures related to the electron-boson interaction, from which the charact...

The energy scale Ω0 of the electron-boson spectral function in the heavy-fermion, d-wave superconductor NpPd5Al2 is predicted on the basis of Eliashberg theory calculations. Assuming a spectral function shape typical for antiferromagnetic spin fluctuations, and imposing constraints provided by the experimental values for the critical temperature an...

Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Omega_b(...

Superconducting and normal-state properties of MgB2 polycrystalline samples with non magnetic (SiC) and magnetic (Co) dopant addition were analysed in order to study the doping influence on the magnetic anisotropy of MgB2 polycrystalline samples and to correlate this influence with the doping-induced changes in band scattering processes. Both dopin...