arXiv subjects

Giovanni A. Ummarino

Publications and source records attributed to Giovanni A. Ummarino.

15 recordsLinked to original sources

arXiv2026

Ginzburg-Landau Theory for Confined Thin-Film Superconductors

We develop a Ginzburg-Landau theory for superconducting thin films under quantum confinement. Starting from the Ginzburg-Landau theory and the recently developed confinement theory of metallic thin films, explicit analytical expressions are derived for the coherence length, penetration depth, electronic mean free path, and Ginzburg-Landau parameter in confined geometries. The central result is that quantum confinement directly renormalizes the intrinsic superconducting coherence length through confinement-induced modifications of the electronic density of states and Fermi energy. This effect is absent in conventional thin-film transport theories based solely on surface scattering. As a consequence, confinement simultaneously suppresses the coherence length and enhances the penetration depth, thereby driving superconductors toward stronger type-II behavior over a broad thickness range. The interplay between quantum confinement and disorder can also produce reentrant type-I/type-II crossovers. The theory predicts a crossover regime in which confinement-induced renormalization of superconducting length scales and transport scattering become strongly intertwined. Comparison with recent penetration-depth measurements in Al thin films shows that the observed enhancement of the penetration depth originates from the interplay between confinement-induced renormalization of the coherence length and suppression of the effective mean free path by surface and disorder scattering. The results establish a direct connection between quantum confinement and superconducting electrodynamics in confined metallic films.

cond-mat.supr-con↗

arXiv2026

Turning non-superconducting elements into superconductors by quantum confinement and proximity

Elemental good metals, including noble metals (Cu, Ag, Au) and several $s$-block elements, do not exhibit superconductivity in bulk at ambient pressure, mainly due to weak electron-phonon coupling that cannot overcome Coulomb repulsion. Quantum confinement in ultra-thin films reshapes the electronic spectrum and the density of states near the Fermi level, producing strong, often non-monotonic, thickness dependencies of the critical temperature in established superconductors. Here, we examine whether confinement alone, or combined with proximity effects, can induce superconductivity in metals that are non-superconducting in bulk form. We review recent theoretical progress and introduce a unified framework based on a confinement-generalized, isotropic one-band Eliashberg theory, where the normal density of states becomes energy dependent and key parameters ($E_F$, $λ$, $μ^$) acquire explicit thickness dependence. By numerically solving the Eliashberg equations using ab initio or experimentally determined electron-phonon spectral functions $α^2F(Ω)$ and Coulomb pseudopotentials $μ^$, and without adjustable parameters, we compute the critical temperature $T_c$ as a function of film thickness for representative noble, alkali, and alkaline-earth metals. The results predict that superconductivity emerges only in selected cases and within extremely narrow thickness windows, typically at sub-nanometer scales ($L \sim 0.4-0.6$ nm), indicating strong fine-tuning requirements for confinement-induced superconductivity in good metals. We also consider layered superconductor/normal-metal systems where confinement and proximity effects coexist. In these heterostructures, a substantial enhancement of the critical temperature is predicted, even when the constituent materials are non-superconducting or weak superconductors in bulk form.

cond-mat.supr-con↗

arXiv2026

Prediction of new superconducting bilayers heterostructures using quantum confinement and proximity effects

A central challenge in nanoscale superconductivity is to understand and exploit the combined action of quantum confinement and proximity effects in experimentally realistic metallic heterostructures. We theoretically investigate superconducting bilayer heterostructures in which these two effects coexist. Using a generalized Eliashberg framework that incorporates both quantum confinement and proximity coupling, we show that their interplay can substantially enhance the superconducting critical temperature. In particular, the theory predicts superconductivity in selected bilayers whose constituent materials are nonsuperconducting or only weakly superconducting in the bulk. These results identify quantum-confined bilayers as a promising route to engineering emergent superconductivity in metallic heterostructures.

cond-mat.supr-con↗

arXiv2025

Direct evidence for two-gap superconductivity in hydrogen-intercalated titanium diselenide

Transition-metal dichalcogenides (TMDs) offer an extremely rich material platform in the exploration of unconventional superconductivity. The unconventional aspects include exotic coupling mechanisms such as the Ising pairing, a complex interplay with other electronic orders such as charge-density waves (CDWs), symmetry-breaking and topological effects, and non-trivial gap structures such as multi-gap and possible nodal phases. Among TMDs, titanium diselenide (1$T$-TiSe$_2$) is one of the most studied and debated cases. Hints to an anomalous structure of its superconducting order parameter have emerged over the years, possibly linked to its spatial texturing in real and reciprocal space due to the presence of a 2$\times$2$\times$2 CDW phase, or to a pressure-driven multi-band Fermi surface. However, a direct evidence for a non-trivial structure of the superconducting gap in this material is still lacking. In this work, we bring the first evidence for a two-gap structure in the recently-discovered H-intercalated TiSe$_2$ superconductor (with T$_c \simeq 3.6$ K) by an extensive experimental study that combines magnetotransport measurements, point-contact spectroscopy and scanning tunnel spectroscopy. We show that the temperature dependence of the upper critical field (for $\vec B \parallel c$) as well as the shape of the point-contact and tunneling spectra strongly suggest the existence of two distinct superconducting gaps, and can indeed all be fitted in a self-consistent way with the same gap amplitudes $Δ_1 = 0.26 \pm 0.12$ meV and $Δ_2 =0.62 \pm 0.18$ meV.

cond-mat.supr-con↗

arXiv2025

Microscopic mechanism of electric field-induced superconductivity suppression in metallic thin films

Supercurrent field-effect transistors made from thin metallic films are a promising option for next-generation high-performance computation platforms. Despite extensive research, there is still no complete quantitative microscopic explanation for how an external DC electric field suppresses superconductivity in thin films. This study aims to provide a quantitative description of superconductivity as a function of film thickness based on Eliashberg's theory. The calculation considers the electrostatics of the electric field, its realistic penetration depth in the film, and its effect on the Cooper pair, which is described as a standard s-wave bound state according to BCS theory. The estimation suggests that an external electric field of approximately $10^8$ V/m is required to suppress superconductivity in 10-30-nm-thick films, which aligns with experimental observations. Ultimately, the study offers "materials by design" guidelines for suppressing supercurrent when an external electric field is applied to the film surface. Furthermore, the proposed framework can be easily extended to investigate the same effects for ultrathin films.

cond-mat.supr-con↗

arXiv2025

Can the noble metals (Au, Ag and Cu) be superconductors?

It is common knowledge that noble metals are excellent conductors but do not exhibit superconductivity. On the other hand, quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals. We present a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness $L$. This parameter-free theory predicts a maximum increase in the critical temperature for a specific value of the film thickness, which is a function of the number of free carriers in the material. Exploiting this fact, we predict that ultra-thin films of gold, silver and copper of suitable thickness could be superconductors at low but experimentally accessible temperatures. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer.

cond-mat.supr-con↗

arXiv2024

Optical and transport properties of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films

The broad-band optical spectroscopy was used to study the optical and the hidden transport properties of the Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ superconducting films with different Ni contents. The normal state data were analyzed using a Drude-Lorentz model with two Drude components: narrow and broad ones. In the superconducting state, two gaps with $2Δ_{0}^{(1)}/k_{B}T_{c}=1.57$ and $2Δ_{0}^{(2)}/k_{B}T_{c}=3.48$ are formed for the Ba(Fe$_{0.965}$Ni$_{0.035}$)$_2$As$_2$ films, while for the Ba(Fe$_{0.95}$Ni$_{0.05}$)$_2$As$_2$ films these characteristic ratios are 1.88--2.08 and 3.66--4.13. Both gaps are formed from the narrow Drude component, whereas the broad Drude component remains ungapped. The calculated from infrared data total dc resistivity of the films with Ni contents $x=0.05$ and $x=0.08$ as well as the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. On the contrary, the films with $x=0.035$ manifest a hidden non-Fermi-liquid behavior. The Allen theory generalized to a multiband systems was applied to the analysis of the temperature dependences of a resistivity of the Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films. The change of total electron-boson coupling and representative energy in the normal state versus the superconducting state for this system was shown to be typical of other iron-based superconducting materials as well as high-temperature superconducting cuprates.

cond-mat.supr-con↗

arXiv2024

THz optical response of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films analyzed within the three-band Eliashberg s$_\pm $-wave model

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ is a multiband superconductor with s$_\pm $-wave pairing symmetry.

cond-mat.supr-con↗

arXiv2024

Pressure weakens coupling strength in In and Sn elemental superconductors

Pressure dependence of the thermodynamic critical field $B_{\rm c}$ in elemental indium (In) and tin (Sn) superconductors was studied by means of the muon-spin rotation/relaxation. Pressure enhances the deviation of $B_{\rm c}(T)$ from the parabolic behavior, expected for a typical type-I superconductor, suggesting a weakening of the coupling strengths $α=\langleΔ\rangle /k_{\rm B}T_{\rm c}$ ($\langleΔ\rangle$ is the average value of the superconducting energy gap, $T_{\rm c}$ is the transition temperature and $k_{\rm B}$ is the Boltzmann constant). As pressure increases from 0.0 to $\simeq 3.0$ GPa $α$ decreases linearly, by approaching the limiting weak-coupling BCS value $α_{\rm BCS}=1.764$. Analysis of the data within the framework of the Eliashberg theory reveals that only part of the pressure effect on $α$ can be attributed to the effect of hardening of the phonon spectra, which is reflected by a decrease of the electron-phonon coupling constant. Nearly 40% of the effect is caused by increased anisotropy of the superconducting energy gap.

cond-mat.supr-con↗

arXiv2024

Gravitational effects in a superconducting film struck by a laser pulse

We study the local interaction of the gravitational field with a superfluid condensate. To this end, we exploit the Ginzburg-Landau formalism with generalized Maxwell fields. The analysis shows that a slight local alteration of the gravitational field in a thin superconducting film can be achieved by laser pulses with particular characteristics.

gr-qc↗

arXiv2023

Constrains on s and d components of electron boson coupling constants in one band d wave Eliashberg theory for high Tc superconductors

The phenomenology of overdoped high Tc superconductors can be described by a one band d wave Eliashberg theory where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations and whose characteristic energy Omega0 scales with Tc according to the empirical law Omega0 = 5.8 kBTc. This model presents universal characteristics that are independent of the critical temperature such as the link between the s and d components of electron boson coupling constants and the invariance of the ratio 2Delta/kBTc.

cond-mat.supr-con↗

arXiv2022

Migdal-Eliashberg theory of multi-band high-temperature superconductivity in field-effect-doped hydrogenated (111) diamond

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$_{\text{c}}$ superconducting phase when the system is field-effect doped to a surface hole concentration of $6\times10^{14}\,$cm$^{-2}$, where the Fermi level crosses three valence bands. Starting from the band-resolved electron-phonon spectral functions $α^2F_{jj'}(ω)$ computed ab initio, we iteratively solve the self-consistent isotropic Migdal-Eliashberg equations, in both the single-band and the multi-band formulations, in the approximation of a constant density of states at the Fermi level. In the single-band formulation, we find T$_{\text{c}}\approx40\,$K, which is enhanced between $4\%$ and $8\%$ when the multi-band nature of the system is taken into account. We also compute the multi-band-sensistive quasiparticle density of states to act as a guideline for future experimental works.

cond-mat.supr-con↗

arXiv2022

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1-xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d-d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d-d model.

cond-mat.supr-con↗

arXiv2020

Superconducting Transition Temperature Modulation in NbN via EDL Gating

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 transition temperature (either positive or negative depending on the sign of the gate voltage) whose magnitude and sign are incompatible with the confinement of the perturbed superconducting state to a thin surface layer, as would be expected within a naïve screening model.

cond-mat.supr-con↗

arXiv2000

Real-Axis Solution of Eliashberg Equations in Various Order-Parameter Symmetries and Tunneling Conductance of Optimally-Doped HTSC

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, anisotropic s and extended s). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Delta, broadening of the gap peak, zero bias and so on) can be explained in the framework of the Migdal-Eliashberg theory. The theoretical dI/dV curves calculated for the different symmetries at T=4 K are then compared to various experimental tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sensitivity of this procedure in determining the exact gap symmetry of these materials. Finally, the effect of the temperature on the theoretical tunneling conductance is also discussed and the curves obtained at T=2 K are compared to those given by the analytical continuation of the imaginary-axis solution.

cond-mat.supr-con↗