We argue that there is a special doping point in the phase diagram of cuprates, such that the condensation of holes into a charge-ordered and a superconducting phase are degenerate in energy but with an energy barrier in between. We present Monte Carlo simulations of a phenomenological XXZ model for this problem without and with quenched disorder in two dimensions. While in the clean case charge order and superconductivity are separated by a first-order line which is nearly independent of temperature, in the presence of quenched disorder charge order is fragmented into domains separated by superconducting filaments reminiscent of the supersolid behaviour in 4He. Assuming weak interlayer couplings, the resulting phase diagram of the three-dimensional system is in good agreement with the experiments

In weakly coupled BCS superconductors, only electrons within a tiny energy window around the Fermi energy, EF, form Cooper pairs. This may not be the case in strong coupling superconductors such as cuprates, FeSe, SrTiO3 or cold atom condensates where the pairing scale, EB, becomes comparable or even larger than EF. In cuprates, for example, a plausible candidate for the pseudogap state at low doping is a fluctuating pair density wave, but no microscopic model has yet been found which supports such a state. In this work, we write an analytically solvable model to examine pairing phases in the strongly coupled regime and in the presence of anisotropic interactions. Already for moderate coupling we find an unusual finite temperature phase, below an instability temperature Ti, where local pair correlations have non-zero center-of-mass momentum but lack long-range order. At low temperature, this fluctuating pair density wave can condense either to a uniform d-wave superconductor or the widely postulated pair-density wave phase depending on the interaction strength. Our minimal model offers a unified framework to understand the emergence of both fluctuating and long range pair density waves in realistic systems.

Increasing experimental evidence suggests the occurrence of filamentary superconductivity in different (quasi) two-dimensional physical systems. In this piece of work, we discuss the proposal that under certain circumstances, this occurrence may be related to the competition with a phase characterized by charge ordering in the form of charge-density waves. We provide a brief summary of experimental evidence supporting our argument in two paradigmatic classes of materials, namely transition metal dichalcogenides and cuprates superconductors. We present a simple Ginzburg-Landau two-order-parameters model as a starting point to address the study of such competition. We finally discuss the outcomes of a more sophisticated model, already presented in the literature and encoding the presence of impurities, and how it can be further improved in order to really address the interplay between charge-density waves and superconductivity and the possible occurrence of filamentary superconductivity at the domain walls between different charge-ordered regions.

We generalize the dynamical phase diagram of a Bardeen-Cooper-Schrieffer condensate, considering attractive to repulsive, i.e., critical quenches (CQs) and a nonconstant density of states (DOS). We show that different synchronized Higgs dynamical phases can be stabilized, associated with singularities in the DOS and different quench protocols. In particular, the CQ can stabilize an overlooked high-frequency Higgs dynamical phase related to the upper edge of the fermionic band. For a compensated Dirac system we find a Dirac-Higgs mode associated with the cusp singularity at the Fermi level, and we show that synchronized phases become more pervasive across the phase diagram. The relevance of these remarkable phenomena and their realization in ensembles of fermionic cold atoms confined in optical lattices is also discussed.

Recently it was discovered that, under elevated pressures, MoB2 exhibits superconductivity at a critical temperature Tc as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure R¯3m structure to an MgB2-like P6/mmm structure. This suggests that remarkably high Tc values among diborides are not restricted to MgB2 as previously appeared to be the case, and that similarly high Tc values may occur in other diborides if they can be coerced into the MgB2 structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the R¯3m at high temperatures across the Nb1-xMoxB2 series. X-ray diffraction confirms that the synthesized Nb-substituted MoB2 adopts the MgB2 crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that NbxMo1-xB2 exhibits superconductivity with Tc as high as 8 K and critical fields approaching 6 T.

The Eliashberg theory of superconductivity accounts for the fundamental physics of conventional superconductors, including the retardation of the interaction and the Coulomb pseudopotential, to predict the critical temperature Tc. McMillan, Allen, and Dynes derived approximate closed-form expressions for the critical temperature within this theory, which depends on the electron-phonon spectral function ?2F(?). Here we show that modern machine-learning techniques can substantially improve these formulae, accounting for more general shapes of the ?2F function. Using symbolic regression and the SISSO framework, together with a database of artificially generated ?2F functions and numerical solutions of the Eliashberg equations, we derive a formula for Tc that performs as well as Allen-Dynes for low-Tc superconductors and substantially better for higher-Tc ones. This corrects the systematic underestimation of Tc while reproducing the physical constraints originally outlined by Allen and Dynes. This equation should replace the Allen-Dynes formula for the prediction of higher-temperature superconductors.

DTT, Divertor Tokamak Test facility, is one of the largest nuclear fusion facility under construction in Europe after ITER. Its mission is to provide an integrated nuclear fusion environment where to test power exhaust strategies useful for the first nuclear fusion power plant. It is a fully superconducting tokamak capable of confining deuterium plasmas with high flexibility with respect to shaping and strike point sweeping. Maximum plasma current of Ip = 5.5 MA and toroidal magnetic field of 6 T at the plasma center makes DTT in a position relevant for the present DEMO design. In late 2019, a consortium has been established with the aim at translating the theoretical and technological knowledge of the partners in the design, in the construction and subsequent experimental management and implementation of the Divertor Tokamak Test machine. To the DTT consortium have finally adhered ten partners representing the leading laboratories and universities involved in nuclear fusion in Italy, and the largest Italian oil company. One of the first activities concerned the development and start-up of the project coordination structure. Subsequently the Integrated Project Team started the engineering design phase with the aim at keeping the maturity of the different DTT systems and components so to guarantee the respect of the challenging schedule of construction. This effort led the DTT team to the completion of the design of the systems in critical path in late 2021 and hence to the start of procurement activities. The paper reports the main resulting final design and technical solutions for the systems of the tokamak area whose procurement activities have started or are about to start, and gives an overview of the forthcoming activities.

We study the electronic structure of a superconducting composition of 122-type Fe-based pnictide material, CaFe1.9Co0.1As2 employing high resolution angle resolved photoemission spectroscopy technique. The experimental results exhibit three bands close to Fermi level at "-point of the Brillouin zone among which only one band crosses the Fermi level. In the parent compound, CaFe2As2, all the three bands cross the Fermi level and form three hole pockets. While the destruction of Fermi pockets due to electron doping (Co-substitution dopes electrons into the system) is expected, we observe significant orbital selective band renormalization with respect to the parent compound. It appears that the effect of spin-orbit coupling is stronger in the doped compound.

BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and (01¯0) faces. The Dirac cone on (010) surface is intense and appears at 0.66 eV binding energy. These states have drawn much attention due to contradictory reports on dimensionality and the momentum of these Dirac fermions. We have studied the properties of these Dirac fermions using varied photon energies and different experimental conditions. The behavior of the Dirac cone is found to be two-dimensional. In addition, we found few more surface states appearing at higher binding energies compared to the Dirac cone.

We study quantum effects of the vacuum light-matter interaction in materials embedded in optical cavities. We focus on the electronic response of a two-dimensional semiconductor placed inside a planar cavity. By using a diagrammatic expansion of the electron-photon interaction, we describe signatures of light-matter hybridization characterized by large asymmetric shifts of the spectral weight at resonant frequencies. We follow the evolution of the light dressing from the cavity to the free-space limit. In the cavity limit, light-matter hybridization results in a modification of the optical gap with sizable spectral weight appearing below the bare gap edge. In the limit of large cavities, we find a residual redistribution of spectral weight which becomes independent of the distance between the two mirrors. We show that the photon dressing of the electronic response can be fully explained by using a classical description of light. The classical description is found to hold up to a strong coupling regime of the light-matter interaction highlighted by the large modification of the photon spectra with respect to the empty cavity. We show that, despite the strong coupling, quantum corrections are negligibly small and weakly dependent on the cavity confinement. As a consequence, in contrast to the optical gap, the single-particle electronic band gap is not sensibly modified by strong coupling. Our results show that quantum corrections are dominated by off-resonant photon modes at high energy. As such, cavity confinement can hardly be seen as a knob to control the quantum effects of the light-matter interaction in vacuum.

Coupling among conduction electrons (e.g., Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor CaFe1.9Co0.1As2 (CaCo122) in the CaFe2As2 (Ca122) family employing high-resolution hard x-ray photoemission spectroscopy. While As core levels show almost no change with doping and cooling, the Ca 2p peak of CaCo122 shows reduced surface contribution relative to Ca122 and a gradual shift of the peak position towards lower binding energies with cooling. In addition, we discover the emergence of a feature at the lower binding energy side of the well-screened Fe 2p signal in CaCo122. The intensity of this feature grows with cooling and indicates additional channels to screen the core holes. The evolution of this feature in the superconducting composition and its absence in the parent compound suggests relevance of the underlying interactions in the ground state properties of this class of materials. These results reveal another dimension in the studies of Fe-based superconductors and the importance of such states in the unconventional superconductivity in general.

In this work the results of two Lidar measurements in the infrared range with a Superconducting Nanowire Single Photon Detector (SNSPD) are presented. In the first measurement, performed at the wavelength of 1550 nm, we determined the distance of two hard targets. This result was used to calibrate the experimental setup and to test the readout electronics. Then, we combined the SNSPD with the Lidar setup MALIA (Multiwavelength Lidar Apparatus) to measure the atmospheric aerosol profile at 1064 nm during a cloudy day. For the last measurement the backscattering coefficient was also calculated.

Evidence is reported that topological effects in graph-shaped arrays of superconducting islands can condition superconducting energy gap and transition temperature. The carriers giving rise to the new phase are couples of electrons (Cooper pairs) which, in the superconducting state, behave as predicted for bosons in our structures. The presented results have been obtained both on star and double comb-shaped arrays and the coupling between the islands is provided by Josephson junctions whose potential can be tuned by external magnetic field or temperature. Our peculiar technique for probing distribution on the islands is such that the hopping of bosons between the different islands occurs because their thermal energy is of the same order of the Josephson coupling energy between the islands. Both for star and double comb graph topologies the results are in qualitative and quantitative agreement with theoretical predictions. ? 2021 by the authors. Licensee MDPI, Basel, Switzerland.

The possibility to tune, through the application of a control gate voltage, the supercon-ducting properties of mesoscopic devices based on Bardeen-Cooper-Schrieffer metals was recently demonstrated. Despite the extensive experimental evidence obtained on different materials and geometries, a description of the microscopic mechanism at the basis of such an unconventional effect has not been provided yet. This work discusses the technological potential of gate control of superconductivity in metallic superconductors and revises the experimental results, which provide information regarding a possible thermal origin of the effect: first, we review experiments performed on high-critical-temperature elemental superconductors (niobium and vanadium) and show how devices based on these materials can be exploited to realize basic electronic tools, such as a half-wave rectifier. Second, we discuss the origin of the gating effect by showing gate-driven suppression of the supercurrent in a suspended titanium wire and by providing a comparison between thermal and electric switching current probability distributions. Furthermore, we discuss the cold field-emission of electrons from the gate employing finite element simulations and compare the results with experimental data. In our view, the presented data provide a strong indication regarding the unlikelihood of the thermal origin of the gating effect.

We have studied the effects of optical-frequency light on proximitized InAs/Al Josephson junctions based on highly n-doped InAs nanowires at varying incident photon flux and at three different photon wavelengths. The experimentally obtained IV curves were modeled using a resistively shunted junction model which takes scattering at the contact interfaces into account. Despite the fact that the InAs weak link is photosensitive, the Josephson junctions were found to be surprisingly robust, interacting with the incident radiation only through heating, whereas above the critical current our devices showed non-thermal effects resulting from photon exposure. Our work indicates that Josephson junctions based on highly-doped InAs nanowires can be integrated in close proximity to photonic circuits. The results also suggest that such junctions can be used for optical-frequency photon detection through thermal processes by measuring a shift in critical current.

A crucial issue in cuprates is the extent and mechanism of the coupling of the lattice to the electrons and the superconductivity. Here we report Cu K edge extended X-ray absorption fine structure measurements elucidating the internal quantum tunneling polaron (iqtp) component of the dynamical structure in two heavily overdoped superconducting cuprate compounds, tetragonal YSr2Cu2.75Mo0.25O7.54 with superconducting critical temperature, T-c = 84 K and hole density p = 0.3 to 0.5 per planar Cu, and the tetragonal phase of Sr2CuO3.3 with T-c = 95 K and p = 0.6. In YSr2Cu2.75Mo0.25O7.54 changes in the Cu-apical O two-site distribution reflect a sequential renormalization of the double-well potential of this site beginning at T-c, with the energy difference between the two minima increasing by similar to 6 meV between T-c and 52 K. Sr2CuO3.3 undergoes a radically larger transformation at T-c, >1- angstrom displacements of the apical O atoms. The principal feature of the dynamical structure underlying these transformations is the strongly anharmonic oscillation of the apical O atoms in a double-well potential that results in the observation of two distinct O sites whose Cu-O distances indicate different bonding modes and valence-charge distributions. The coupling of the superconductivity to the iqtp that originates in this nonadiabatic coupling between the electrons and lattice demonstrates an important role for the dynamical structure whereby pairing occurs even in a system where displacements of the atoms that are part of the transition are sufficiently large to alter the Fermi surface. The synchronization and dynamic coherence of the iqtps resulting from the strong interactions within a crystal would be expected to influence this process.

Bound states in superconductor-nanowire hybrid devices play a central role, carrying information on ground-state properties (Shiba or Andreev states) or on the topological properties of the system (Majorana states). The spectroscopy of such bound states relies on the formation of well-defined tunnel barriers, usually defined by gate electrodes, which results in smooth tunnel barriers. Here we used thin InP segments embedded into InAs nanowire during the growth process to form a sharp built-in tunnel barrier. Gate dependence and thermal-activation measurements are used to confirm the presence and estimate the height of this barrier. By coupling these wires to superconducting electrodes we investigate the gate-voltage dependence of the induced gap in the nanowire segment, which we can understand using a simple model based on Andreev bound states. Our results show that these built-in barriers are promising as future spectroscopic tools.

We present here an innovative photon detector based on the proximity junction array device (PAD) working at long wavelengths. We show that the vortex dynamics in PAD undergoes a transition from a Mott insulator to a vortex metal state by application of an external magnetic field. The PAD also evidences a Josephson I-V characteristic with the external field dependent tunneling current. At high applied currents, we observe a dissipative regime in which the vortex dynamics is dominated by the quasi-particle contribution from the normal metal. The PAD has a relatively high photo-response even at frequencies below the expected characteristic frequency while, its superconducting properties such as the order parameter and the Josephson characteristic frequency can be modulated via external fields to widen the detection band. This device represents a promising and reliable candidate for new high-sensitivity long-wavelength detectors.

In the absence of dissipation a periodically driven BCS superconductor can enter a coherent nonlinear regime of collective Rabi oscillations which last for arbitrary long times [Ojeda Collado et al., Phys. Rev. B 98, 214519 (2018)2469-995010.1103/PhysRevB.98.214519]. Here we show that dissipation effects introduce dramatic changes: (i) The collective Rabi mode becomes a transient. (ii) At long times a steady state is reached showing strong nonlinear effects for large enough drive strength. We identify the physical parameters governing the various crossovers and present a detailed computation of time- and angle-resolved photoemission spectroscopy (tr-ARPES) and time-resolved tunneling spectra aiming at detecting the collective Rabi oscillations and the steady-state nonlinearities. We show also that second harmonic generation is allowed for a drive which acts on the BCS coupling constant.

Hole-doped perovskite bismuthates such as Ba1-xKxBiO3 and Sr1-xKxBiO3 are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO3 on SrO-terminated SrTiO3 (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature codeposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO3 has a pseudocubic lattice constant approximate to 4.25 angstrom and an approximate to 8.8% lattice mismatch on SrTiO3 substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature codeposition, the recrystallization method can produce higher quality 1- to 6-unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.