Shanenko, Arkady Arkadyevich

Ding S., Bai Y., Bulekov A.A., Zhang W., Shanenko A.A., Chen Y.

Neverov V.D., Lukyanov A.E., Krasavin A.V., Shanenko A.A., Croitoru M.D., Vagov A.

Chen Y., Chen K., Zhu J., Shanenko A.A.

de Araujo Sarmento M., Cordoba W.Y., Shanenko A., Vagov A., Aguiar J.A., Stolyarov V.S.

Abstract To describe the way complexity emerges in seemingly simple systems of nature, requires one to attend to two principal questions: how complex patterns appear spontaneously and why a single system can accommodate their inexhaustible variety. It is commonly assumed the pattern formation phenomenon is related to the competition of several types of interactions with disparate length scales. These multi-scale interactions also lead to frustration within the system, resulting in the existence of a manifold of configurations-patterns with qualitatively distinct morphologies. This work explores an alternative approach through a mechanism that leads to a wide range of intricate and topologically non-trivial patterns. The mechanism is described by the self-dual Ginzburg-Landau theory and, possibly, other Maxwell–Higgs models. It gives rise to unique spatial flux and condensate spatial profiles observed in superconductors between the two conventional superconductivity types I and II.

Krasavin A.V., Vagov A.V., Vasenko A.S., Stolyarov V.A., Shanenko A.A.

The combination of strongly coupled Cooper pairs and weak superconducting fluctuations is an important prerequisite for achieving high-temperature superconductivity. The review is devoted to the implementation of this condition in multiband superconductors, in which strongly coupled pairs in the shallow conduction band (the Fermi level is close to the band edge) coexist with ordinary, weakly fluctuating Cooper pairs formed in the deep band. As a result of the Josephson coupling between condensates in different bands, such a system is characterized by a high critical coherence temperature due to the presence of strongly coupled pairs and the suppression of superconducting fluctuations. This suppression does not require any special preconditions, and is almost total even if the Josephson coupling between the bands is weak.

Marychev P.M., Shanenko A.A., Vagov A.V.

Nonmagnetic impurity scattering is known to shift up the Ginzburg–Landau parameter κ of a superconductor. In this case, when the system is initially in type I, it can change its magnetic response, crossing the intertype domain with κ ∼ 1 between the two standard superconductivity types and arriving at type II. In the present work we demonstrate that the impact of disorder can be much more profound in the presence of the multiband structure of the charge carrier states. In particular, when the band diffusivities differ from each other, the intertype domain tends to expand significantly, including points with κ ≫ 1 that belong to deep type-II in conventional single-band superconductors. Our finding sheds light on the nontrivial disorder effect and significantly complements earlier results on the enlargement of the intertype domain in clean multiband superconductors.

Krasavin A.V., Vagov A.V., Vasenko A.S., Stolyarov V.S., Shanenko A.A.

An Erratum to this paper has been published: https://doi.org/10.1134/S0021364024020012

Chen Y., Zhu Q., Zhang M., Luo X., Shanenko A.A.

Recently, a surface superconductor-insulator transition has been predicted for a bulk superconductor in an electric field applied perpendicular to its surface. The related calculations were performed within a one-dimensional Hubbard model by numerically solving the Bogoliubov-de Gennes (BdG) equations without the Hartree-Fock (HF) interaction potential. The phase diagram of the surface superconducting, metallic, and insulating states was obtained as dependent on the electric field and temperature. This diagram was found to be in agreement with experimental results reported previously for (Li,Fe)OHFeSe thin flakes. In the present work, by taking into account the HF potential, we find that the latter acts as a kind of an extra electrostatic potential that enhances the electric-field effects on the surface states. The qualitative features of the phase diagram remain the same but the surface superconductor-insulator transition occurs at significantly lower electric fields, which supports prospects of its experimental observation in bulk samples.

Bai Y., Zhang L., Luo X., Shanenko A.A., Chen Y.

Nucleation of the pair condensate near surfaces above the upper critical magnetic field and the pair-condensate enhancement/suppression induced by changes in the electron-phonon interaction at interfaces are the most known examples of the surface superconductivity. Recently, another example has been reported when the surface enhancement of the critical superconducting temperature occurs due to quantum interference. In this case the pair states spread over the entire volume of the system while exhibiting the constructive interference near the surface. In the present work we investigate how an applied electric field impacts the interference-induced surface superconductivity. The study is based on a numerical solution of the self-consistent Bogoliubov-de Gennes equations for a one-dimensional attractive Hubbard model. Our results demonstrate that the surface superconducting characteristics, especially the surface critical temperature, are sensitive to the applied electric field and can be tailored by changing its magnitude.

Vagov A., Saraiva T.T., Shanenko A.A., Vasenko A.S., Aguiar J.A., Stolyarov V.S., Roditchev D.

AbstractIn many pnictides the superconductivity coexists with ferromagnetism in an accessible range of temperatures and compositions. Recent experiments revealed that when the temperature of magnetic ordering Tm is below the superconducting transition temperature Tc, highly non-trivial physical phenomena occur. In this work we demonstrate the existence of a temperature window, situated between Tm and Tc, where these intrinsically type-II superconductors are in the intertype regime. We explore analytically and numerically its rich phase diagram characterized by exotic spatial flux configurations—vortex clusters, chains, giant vortices and vortex liquid droplets—which are absent in both type-I and type-II bulk superconductors. We find that the intertype regime is almost independent of microscopic parameters, and can be achieved by simply varying the temperature. This opens the route for experimental studies of the intertype superconductivity scarcely investigated to date.

Yin L., Bai Y., Zhang M., Shanenko A.A., Chen Y.

It is well known that the electric field can induce phase transitions between superconducting, metallic and insulating states in thin-film materials due to its control of the charge carrier density. Since a similar effect on the charge carriers can also be expected for surfaces of bulk samples, here, we investigate the transformation of the surface states in a superconductor under an applied screened electric field. Our study is performed by numerically solving the self-consistent Bogoliubov--de Gennes equations for the one-dimensional attractive Hubbard model. It is found that the surface insulating regime occurs at sufficiently large (but still experimentally accessible) electric fields. Our calculations yield the phase diagram of the surface superconducting, metallic, and insulating states for a wide range of temperatures and applied fields. Our results are in qualitative agreement with the phase diagram obtained with the transport measurements for (Li, Fe)OHFeSe thin flakes [Ma et al., Sci. Bull. 64, 653 (2019); Yin et al., ACS Nano 14, 7513 (2020)].

de Bragança R.H., Croitoru M.D., Shanenko A.A., Aguiar J.A.

Bai Y., Chen Y., Croitoru M.D., Shanenko A.A., Luo X., Zhang Y.

In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to $20--25%$ as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interference-induced enhancement can be much more pronounced, up to nearly $70%$. Furthermore, here it is shown that such an interference enhancement persists over a wide range of microscopic parameters.

Chen Y., Shanenko A.A.

The interference of multiple condensates coexisting in one system may lead to unconventional coherent behavior. This is expected when the spatial lengths of the condensates are essentially different. Traditionally, the characteristic spatial length of a superconducting condensate is associated with the gap function. However, the broader readership is more familiar with the concept of the Cooper-pair wave function. For conventional single-band superconductors, the gap function coincides with the center-of-mass Cooper-pair wave function up to the coupling constant, and the corresponding gap and wave function characteristic lengths are the same. Surprisingly, we find that in two-band superconductors, these lengths are the same only near the critical temperature. At lower temperatures, they can significantly deviate from each other, and the fundamental question of which of these lengths should be preferred when specifying the spatial scale of a band-dependent condensate in multiband superconducting materials arises.

Serovaiskii A., Kutcherov V.G., Vinokurov V.A., Serebryakov S.G., Trotsenko V.G., Zhukova E.S., Bush A.A., Shanenko A.A., Vasenko A.S., Stolyarov V.S., Kozlov V.I.

BiScO3 compound was obtained in the form of dense ceramic with a perovskite-type structure, and its complex characterization was determined for the first time. The corresponding synthesis procedure is described in detail. It is demonstrated that the temperature region of the phase stability at atmospheric pressure lies at T < 700 °C (973 K). It is shown that the crystal structure of the BiScO3 ceramic is centrosymmetric. Dielectric measurements of the synthesized sample performed at frequencies 25 Hz to 1 MHz and at temperatures 10-340 K show no changes typical for phase transition. Room-temperature infrared (30-15600 cm-1) and Raman (90-2000 cm-1) spectra of the prepared BiScO3 ceramic are measured, and information on the parameters of phonon resonances is obtained. The number of infrared modes exceeds that predicted by the factor group analysis of the noncentrosymmetric space group C2. The reason for selection rules violation can be associated with the disorder of the crystal structure and local distortions induced by the lone pair of electrons of Bi3+.

Ooi S., Tachiki M., Mochiku T., Ito H., Kubo T., Kikuchi A., Arisawa S., Umemori K.

Alshemi A., Forgan E. ., Hiess A., Cubitt R., White J. ., Schmalzl K., Blackburn E.

Multiband superconductivity arises when multiple electronic bands contribute to the formation of the superconducting state, allowing distinct pairing interactions and gap structures. Here, we present field- and temperature-dependent data on the vortex lattice structure in 2H−NbSe2 as a contribution to the ongoing debate as to whether the defining feature of the superconductivity is the anisotropy or the multiband nature. The field-dependent data clearly show that there are two distinct superconducting bands, and the contribution of one of them to the vortex lattice signal is completely suppressed for magnetic fields above ∼0.8  T, well below Bc2. By combining the temperature and field scans, we can deduce that there is a moderate degree of interband coupling. From the observed temperature dependences, we find that at low field and zero temperature, the two gaps in temperature units are 13.1±0.2 and 6.5±0.3  K (Δ0=1.88 and 0.94 kBTc); the band with the larger gap gives just under two-thirds of the superfluid density. The penetration depth extrapolated to zero field and zero temperature is 160±2  nm. Published by the American Physical Society 2025

Chen K., Hosur P.

Hovhannisyan R.A., Grebenchuk S.Y., Larionov S.A., Shishkin A.G., Grebenko A.K., Kupchinskaya N.E., Dobrovolskaya E.A., Skryabina O.V., Aladyshkin A.Y., Dremov V.V., Golovchanskiy I.A., Samokhvalov A.V., Mel’nikov A.S., Roditchev D., Stolyarov V.S.

Chen P.

Velasco V., Midei G., Capone M., Perali A.

Domínguez D., Berger J.

Polo A.S., Martínez L.F., Ariza Echeverri E.A., Deluque Toro C.E., Faúndez J., Aguirre C., Barba-Ortega J.

In this study, we employed the Pearson product-moment correlation coefficient ([Formula: see text]) to identify non-composite vortex states in a two-band superconducting slab. The fractional vortices were simulated using the two-component Ginzburg–Landau equations TDGL, with the link variable method and the healing coupling between the superconducting bands. The coefficient [Formula: see text] was calculated based on the square modulus of the order parameters of the two condensates, providing insight into the correlation between the two bands. Statistically, [Formula: see text] ranges from [Formula: see text]1 to 1, where values approaching 1 indicate composite vortices, while values nearing 0 suggest non-composite vortices. In this context, negative values of [Formula: see text] are irrelevant. The use of this tool enabled us to assess the degree of decoupling (coincidence of the position of the vortex centers in the superconducting bands) within the vortex lattice, allowing for the identification of specific fields and currents associated with the uncoupled vortex states in the superconducting slab. Such findings are crucial for managing energy dissipation in multi-band superconducting materials and manipulation of the vortices state in multi-band superconducting system.

Wang G., Han T., Li J., Zhang J., Huang H.

Based on two-band time-dependent Ginzburg–Landau theory, we study the electromagnetic properties of mesoscopic type-1.5 superconductors with different defect configurations. We perform numerical simulations with the finite element method, and give direct evidence for the existence of a vortex cluster phase in the presence of nonmagnetic impurity. In addition, we also investigate the depinning critical current of the magnetic vortex cluster induced by the isotropic or anisotropic defect structure under the external current. Our theoretical results thus indicate that the diversity of impurity deposition has a significant influence on the semi-Meissner state in type-1.5 superconductors.

Ji H., Yuan N.F.

Inspired by the recent experiments in monolayer iron-based superconductors, we theoretically investigate the properties of a two-dimensional multiband superconductor under magnetic fields, focusing on two aspects. First, for vortex bound states under out-of-plane magnetic fields, the spatial anisotropy and positions of electron density peaks are associated with interband couplings. Second, under in-plane magnetic fields, even with inversion symmetry, a Ising-type spin-orbit coupling is allowed, leading to an enhanced in-plane upper critical field. Applications to other two-dimensional multiband superconductors are also discussed. Published by the American Physical Society 2025

Brems X.S., Muehlbauer S., Cubitt R.

Abstract The intermediate mixed state under the influence of a transport current was studied using small angle neutron scattering. The internal magnetic domain structure consisting of mixed state domains and flux free Meissner state domains was observed to rearrange at intermediate currents well before the critical depinning current marked by a finite voltage. The local rearrangement can be traced by the changes in the vortex lattice Bragg peak scattering and the current-induced anisotropy of the low-q scattering connected to the internal magnetic domain structure. It is argued, that the local reorganisation prior to the critical depinning current is inherently linked to the interplay of the pinning landscape with the vortex lattice domain structure governed by the physics of the intermediate mixed state.

Deshpande A., Pusskeiler J., Prange C., Rogge U., Dressel M., Scheffler M.

The peculiar superconducting properties of granular aluminum, which consists of nanometer-sized aluminum grains separated by aluminum oxide, are attractive for applications in quantum circuitry, and they are interesting from a fundamental materials physics view. The phase diagram of granular aluminum as a function of normal-state resistivity features a superconducting dome with a maximum critical temperature Tc well above the Tc=1.2K of pure aluminum. Here, we show how the maximum Tc of this superconducting dome grows if the substrate temperature during deposition is lowered from 300 K to cooling with liquid nitrogen (150 and 100 K) and liquid helium (25 K). The highest Tc that we observe is 3.27 K. These results highlight that granular aluminum is a model system for complex phase diagrams of superconductors and demonstrate its potential in the context of high kinetic inductance applications. This is augmented by our observation of comparably sharp superconducting transitions of high-resistivity samples grown at cryogenic temperatures and by a thickness dependence even for films substantially thicker than the grain size.

André Orna T. D., Doria M.M., Reyes D.

Krasavin A.V., Vagov A.V., Vasenk A.S., Stolyarov V.S., Shanenko A.A.

Witt N., Nomura Y., Brener S., Arita R., Lichtenstein A.I., Wehling T.O.

AbstractSuperconductivity emerges from the spatial coherence of a macroscopic condensate of Cooper pairs. Increasingly strong binding and localization of electrons into these pairs compromises the condensate’s phase stiffness, thereby limiting critical temperatures – a phenomenon known as the BCS–BEC crossover in lattice systems. In this study, we demonstrate enhanced superconductivity in a multiorbital model of alkali-doped fullerides (A3C60) that goes beyond the limits of the lattice BCS–BEC crossover. We identify that the interplay of strong correlations and multiorbital effects results in a localized superconducting state characterized by a short coherence length but robust stiffness and a domeless rise in critical temperature with increasing pairing interaction. To derive these insights, we introduce a new theoretical framework allowing us to calculate the fundamental length scales of superconductors, namely the coherence length (ξ0) and the London penetration depth (λL), even in presence of strong electron correlations.

Krasavin A.V., Vagov A.V., Vasenko A.S., Stolyarov V.A., Shanenko A.A.

The combination of strongly coupled Cooper pairs and weak superconducting fluctuations is an important prerequisite for achieving high-temperature superconductivity. The review is devoted to the implementation of this condition in multiband superconductors, in which strongly coupled pairs in the shallow conduction band (the Fermi level is close to the band edge) coexist with ordinary, weakly fluctuating Cooper pairs formed in the deep band. As a result of the Josephson coupling between condensates in different bands, such a system is characterized by a high critical coherence temperature due to the presence of strongly coupled pairs and the suppression of superconducting fluctuations. This suppression does not require any special preconditions, and is almost total even if the Josephson coupling between the bands is weak.

Marychev P.M., Shanenko A.A., Vagov A.V.

Nonmagnetic impurity scattering is known to shift up the Ginzburg–Landau parameter κ of a superconductor. In this case, when the system is initially in type I, it can change its magnetic response, crossing the intertype domain with κ ∼ 1 between the two standard superconductivity types and arriving at type II. In the present work we demonstrate that the impact of disorder can be much more profound in the presence of the multiband structure of the charge carrier states. In particular, when the band diffusivities differ from each other, the intertype domain tends to expand significantly, including points with κ ≫ 1 that belong to deep type-II in conventional single-band superconductors. Our finding sheds light on the nontrivial disorder effect and significantly complements earlier results on the enlargement of the intertype domain in clean multiband superconductors.

Neverov V.D., Kalashnikov A., Lukyanov A.E., Krasavin A.V., Croitoru M.D., Vagov A.

This work introduces an algorithm designed to solve the Bogoliubov–de Gennes equations of superconductivity theory. What sets this algorithm apart is its remarkable ability to precisely and consistently consider the impact of an external magnetic field, all within the microscopic approach. The computation scheme’s convergence is guaranteed by addressing the Biot–Savart equation for the field where the vector potential appears on both of its sides. To showcase the capabilities of this approach, we provide several key examples: the Abrikosov lattice, vortex core states, and the vortex structure in the intermediate mixed state of a superconductor. This method promises to offer valuable insights into the microscopic physics of intertype superconductivity.

Chen Y., Zhu Q., Zhang M., Luo X., Shanenko A.A.

Recently, a surface superconductor-insulator transition has been predicted for a bulk superconductor in an electric field applied perpendicular to its surface. The related calculations were performed within a one-dimensional Hubbard model by numerically solving the Bogoliubov-de Gennes (BdG) equations without the Hartree-Fock (HF) interaction potential. The phase diagram of the surface superconducting, metallic, and insulating states was obtained as dependent on the electric field and temperature. This diagram was found to be in agreement with experimental results reported previously for (Li,Fe)OHFeSe thin flakes. In the present work, by taking into account the HF potential, we find that the latter acts as a kind of an extra electrostatic potential that enhances the electric-field effects on the surface states. The qualitative features of the phase diagram remain the same but the surface superconductor-insulator transition occurs at significantly lower electric fields, which supports prospects of its experimental observation in bulk samples.

Bai Y., Zhang L., Luo X., Shanenko A.A., Chen Y.

Nucleation of the pair condensate near surfaces above the upper critical magnetic field and the pair-condensate enhancement/suppression induced by changes in the electron-phonon interaction at interfaces are the most known examples of the surface superconductivity. Recently, another example has been reported when the surface enhancement of the critical superconducting temperature occurs due to quantum interference. In this case the pair states spread over the entire volume of the system while exhibiting the constructive interference near the surface. In the present work we investigate how an applied electric field impacts the interference-induced surface superconductivity. The study is based on a numerical solution of the self-consistent Bogoliubov-de Gennes equations for a one-dimensional attractive Hubbard model. Our results demonstrate that the surface superconducting characteristics, especially the surface critical temperature, are sensitive to the applied electric field and can be tailored by changing its magnitude.

Xing B., Feng C., Scalettar R., Batrouni G.G., Poletti D.

The Su-Schrieffer-Heeger (SSH) model, with bond phonons modulating electron tunneling, is a paradigmatic electron-phonon model that hosts an antiferromagnetic order to bond order transition at half-filling. In the presence of a repulsive Hubbard interaction, the antiferromagnetic phase is enhanced, but the phase transition remains first order. Here, we explore the physics of the SSH model with an attractive Hubbard interaction, which hosts an interesting interplay among charge order, $s$-wave pairing, and bond order. Using the numerically exact determinant quantum Monte Carlo method, we show that both charge order, present at weak electron-phonon coupling, and bond order, at large coupling, give way to $s$-wave pairing when the system is doped. Furthermore, we demonstrate that the SSH electron-phonon interaction competes with the attractive Hubbard interaction and reduces the $s$-wave pairing correlation.

Vagov A., Saraiva T.T., Shanenko A.A., Vasenko A.S., Aguiar J.A., Stolyarov V.S., Roditchev D.

AbstractIn many pnictides the superconductivity coexists with ferromagnetism in an accessible range of temperatures and compositions. Recent experiments revealed that when the temperature of magnetic ordering Tm is below the superconducting transition temperature Tc, highly non-trivial physical phenomena occur. In this work we demonstrate the existence of a temperature window, situated between Tm and Tc, where these intrinsically type-II superconductors are in the intertype regime. We explore analytically and numerically its rich phase diagram characterized by exotic spatial flux configurations—vortex clusters, chains, giant vortices and vortex liquid droplets—which are absent in both type-I and type-II bulk superconductors. We find that the intertype regime is almost independent of microscopic parameters, and can be achieved by simply varying the temperature. This opens the route for experimental studies of the intertype superconductivity scarcely investigated to date.

Garisto D.

Efforts to replicate the material have pieced together the puzzle of why it displayed superconducting-like behaviours. Efforts to replicate the material have pieced together the puzzle of why it displayed superconducting-like behaviours.

Yin L., Bai Y., Zhang M., Shanenko A.A., Chen Y.

It is well known that the electric field can induce phase transitions between superconducting, metallic and insulating states in thin-film materials due to its control of the charge carrier density. Since a similar effect on the charge carriers can also be expected for surfaces of bulk samples, here, we investigate the transformation of the surface states in a superconductor under an applied screened electric field. Our study is performed by numerically solving the self-consistent Bogoliubov--de Gennes equations for the one-dimensional attractive Hubbard model. It is found that the surface insulating regime occurs at sufficiently large (but still experimentally accessible) electric fields. Our calculations yield the phase diagram of the surface superconducting, metallic, and insulating states for a wide range of temperatures and applied fields. Our results are in qualitative agreement with the phase diagram obtained with the transport measurements for (Li, Fe)OHFeSe thin flakes [Ma et al., Sci. Bull. 64, 653 (2019); Yin et al., ACS Nano 14, 7513 (2020)].

Mizukami Y., Haze M., Tanaka O., Matsuura K., Sano D., Böker J., Eremin I., Kasahara S., Matsuda Y., Shibauchi T.

AbstractThe BCS-BEC (Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate) crossover from strongly overlapping Cooper pairs to non-overlapping composite bosons in the strong coupling limit has been a long-standing issue of interacting many-body fermion systems. Recently, FeSe semimetal with hole and electron bands emerged as a high-transition-temperature (high-Tc) superconductor located in the BCS-BEC crossover regime, owing to its very small Fermi energies. In FeSe, however, an ordinary BCS-like heat-capacity jump is observed at Tc, posing a fundamental question on the characteristics of the BCS-BEC crossover. Here we report on high-resolution heat capacity, magnetic torque, and scanning tunneling spectroscopy measurements in FeSe1−xSx. Upon entering the tetragonal phase at x > 0.17, where nematic order is suppressed, Tc discontinuously decreases. In this phase, highly non-mean-field behaviours consistent with BEC-like pairing are found in the thermodynamic quantities with giant superconducting fluctuations extending far above Tc, implying the change of pairing nature. Moreover, the pseudogap formation, which is expected in BCS-BEC crossover of single-band superconductors, is not observed in the tunneling spectra. These results illuminate highly unusual features of the superconducting states in the crossover regime with multiband electronic structure and competing electronic instabilities.

Iguchi Y., Shi R.A., Kihou K., Lee C., Barkman M., Benfenati A.L., Grinenko V., Babaev E., Moler K.A.

Magnetic field penetrates type-II bulk superconductors by forming quantum vortices that enclose a magnetic flux equal to the magnetic flux quantum. The flux quantum is a universal quantity that depends only on fundamental constants. Here we investigate isolated vortices in the hole-overdoped Ba 1− x K x Fe 2 As 2 ( x = 0.77) by using scanning superconducting quantum interference device (SQUID) magnetometry. In many locations, we observed vortices that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. We interpret these features as quantum vortices with non-universally quantized (fractional) magnetic flux whose magnitude is determined by the temperature-dependent parameters of a multiband superconductor. The demonstrated mobility and manipulability of the fractional vortices may enable applications in fluxonics-based computing.

de Bragança R.H., Croitoru M.D., Shanenko A.A., Aguiar J.A.

Amoretti A.

Abstract A static electric field has always been thought to play little role in the physics of ideal conductors, since the screening effects of mobile carriers prevent it from penetrating deep into the bulk of a metal. Very recently however, experimental evidence has been obtained which indicates that static electric fields can be used to manipulate the superconductive properties of metallic BCS superconducting thin films, weakening the critical current. In this paper I will show how possible explanations to this striking effect can be found relying on the analogy between Superconductivity and Quantum Electrodynamics noticed by Nambu and Iona-Lasinio in the sixties. I will show that, following this parallelism, it is possible to predict a new phenomenon: the superconducting Schwinger effect. Secondly I will explain how this new microscopic effect can be connected to a modified Gizburg-Landau theory where additional couplings between electric field and the superconductive condensate are taken into account. Eventually I will connect these theoretical predictions to the experiments, proposing them as a possible explanation of the weakening of superconductivity due to an external electric field.

Sous J., He Y., Kivelson S.A.

AbstractWe examine key aspects of the theory of the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensation (BEC) crossover, focusing on the temperature dependence of the chemical potential, μ. We identify an accurate method of determining the change of μ in the cuprate high temperature superconductors from angle-resolved-photoemission data (along the ‘nodal’ direction), and show that μ varies by less than a few percent of the Fermi energy over a range of temperatures from far below to several times above the superconducting transition temperature, Tc. This shows, unambiguously, that not only are these materials always on the BCS side of the crossover (which is a phase transition in the d-wave case), but are nowhere near the point of the crossover (where the chemical potential approaches the band bottom).

Lin H., Huang W., Rai G., Yin Y., He L., Xue Q., Haas S., Kettemann S., Chen X., Ji S.

The quantum many-body states in the Bardeen-Cooper-Schrieffer--Bose-Einstein condensation (BCS-BEC) crossover regime are of long-lasting interest. Here we report direct spectroscopic evidence of BCS-BEC crossover in real space in a FeSe monolayer thin film by using spatially resolved scanning tunneling spectra. The crossover is driven by the shift of band structure relative to the Fermi level. The theoretical calculation based on a two-band model qualitatively reproduces the measured spectra in the whole crossover range. In addition, the Zeeman splitting of the quasiparticle states is found to be consistent with the characteristics of a condensate. Our work paves the way to study the exotic states of BCS-BEC crossover in a two-dimensional crystalline material at the atomic scale.