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In multi-orbital materials, superconductivity can exhibit several coupled condensates. In this context, quantum confinement in two-dimensional superconducting oxide interfaces offers new degrees of freedom to engineer the band structure and selectively control the occupancy of 3d orbitals by electrostatic doping. Here, we use resonant microwave transport to extract the superfluid stiffness of the (110)-oriented LaAlO3/SrTiO3 interface in the entire phase diagram. We provide evidence of a transition from single-condensate to two-condensate superconductivity driven by continuous and reversible electrostatic doping, which we relate to the Lifshitz transition between 3d bands based on numerical simulations of the quantum well. We find that the superconducting gap is suppressed while the second band is populated, challenging Bardeen–Cooper–Schrieffer theory. We ascribe this behaviour to the existence of superconducting order parameters with opposite signs in the two condensates due to repulsive coupling. Our findings offer an innovative perspective on the possibility to tune and control multiple-orbital physics in superconducting interfaces.Electrostatic doping drives a transition from single condensate to two condensate superconductivity at the (110)-oriented LaAlO3/SrTiO3 interface.

In the underdoped regime, the cuprate high-temperature superconductors exhibit a host of unusual collective phenomena, including unconventional spin and charge density modulations, Fermi surface reconstructions, and a pseudogap in various physical observables. Conversely, overdoped cuprates are generally regarded as conventional Fermi liquids possessing no collective electronic order. In partial contradiction to this widely held picture, we report resonant X-ray scattering measurements revealing incommensurate charge order reflections for overdoped (Bi,Pb)2.12Sr1.88CuO6+δ (Bi2201), with correlation lengths of 40–60 lattice units, that persist up to temperatures of at least 250 K. The value of the charge order wavevector decreases with doping, in line with the extrapolation of the trend previously observed in underdoped Bi2201. In overdoped materials, however, charge order coexists with a single, unreconstructed Fermi surface without nesting or pseudogap features. The discovery of re-entrant charge order in Bi2201 thus calls for investigations in other cuprate families and for a reconsideration of theories that posit an essential relationship between these phenomena.

In LaAlO 3 /SrTiO 3 heterostructures, a gate tunable superconducting electron gas is confined in a quantum well at the interface between two insulating oxides. Remarkably, the gas coexists with both magnetism and strong Rashba spin–orbit coupling. However, both the origin of superconductivity and the nature of the transition to the normal state over the whole doping range remain elusive. Here we use resonant microwave transport to extract the superfluid stiffness and the superconducting gap energy of the LaAlO 3 /SrTiO 3 interface as a function of carrier density. We show that the superconducting phase diagram of this system is controlled by the competition between electron pairing and phase coherence. The analysis of the superfluid density reveals that only a very small fraction of the electrons condenses into the superconducting state. We propose that this corresponds to the weak filling of high-energy d xz / d yz bands in the quantum well, more apt to host superconductivity. The nature of the doping dependent superconducting transition remains elusive for a two dimensional electron gas at the LaAlO 3 /SrTiO 3 interface. Here, Singh et al. report superfluid stiffness and the superconducting gap energy at such interface as a function of carrier density.

The diverse phenomena associated with the two-dimensional electron gas (2DEG) that occurs at oxide interfaces include, among others, exceptional carrier mobilities, magnetism and superconductivity. Although these have mostly been the focus of interest for potential future applications, they also offer an opportunity for studying more fundamental quantum many-body effects. Here, we examine the magnetic-field-driven quantum phase transition that occurs in electrostatically gated superconducting LaTiO 3 /SrTiO 3 interfaces. Through a finite-size scaling analysis, we show that it belongs to the (2+1)D X Y model universality class. The system can be described as a disordered array of superconducting puddles coupled by a 2DEG and, depending on its conductance, the observed critical behaviour is single (corresponding to the long-range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). A phase diagram illustrating the dependence of the critical field on the 2DEG conductance is constructed, and shown to agree with theoretical proposals. Moreover, by retrieving the coherence-length critical exponent ν , we show that the quantum critical behaviour can be clean or dirty according to the Harris criterion, depending on whether the phase-coherence length is smaller or larger than the size of the puddles. The range of phenomena associated with the two-dimensional electron gas occurring at oxide interfaces has garnered significant attention. By performing a finite-size scaling analysis, the universality class of the magnetic field-driven quantum phase transition that occurs at the superconducting LaTiO 3 /SrTiO 3 interface is now established.

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO 3 /SrTiO 3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO 3 /SrTiO 3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements show that the Rashba coupling constant increases linearly with the interfacial electric field. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.

Constraining the role of dust deposition in regulating the concentration of the essential micronutrient iron in surface ocean waters requires knowledge of the flux of seawater‐soluble iron in aerosols and the replacement time of dissolved iron (DFe) in the euphotic zone. Here we estimate these quantities using seasonally resolved DFe data from the Bermuda Atlantic Time‐series Study region and weekly‐scale measurements of iron in aerosols and rain from Bermuda during 2019. In response to seasonal changes in vertical mixing, primary production and dust deposition, surface DFe concentrations vary from ∼0.2 nM in early spring to >1 nM in late summer, with DFe inventories ranging from ∼30 to ∼80 μmol/m2, respectively, over the upper 200 m. Assuming the upper ocean approximates steady state for DFe on an annual basis, our aerosol and rainwater data require a mean euphotic‐zone residence time of ∼0.8–1.9 years for DFe with respect to aeolian input. Plain Language Summary Primary production by phytoplankton in ocean surface waters is the foundation of the marine ecosystem, and plays a key role in maintaining the ocean‐atmosphere balance of carbon dioxide, which regulates global climate. Iron is an essential micronutrient that is required by phytoplankton, and the availability of dissolved iron (DFe) is thought to limit phytoplankton growth over large areas of the ocean. In this context, it is important to constrain the sources and persistence of DFe in surface ocean waters, which control the amount of DFe that is available to support phytoplankton growth. This study focuses on the Bermuda region of the North Atlantic Ocean, where deposition of airborne soil dust is the major source of DFe to surface waters. By combining measurements of the atmospheric loading and solubility of iron in soil dust over Bermuda with measurements of DFe in adjacent ocean waters over a full year, we are able to estimate the rate of supply of DFe from dust deposition in this region, as well as the average time that this DFe persists in the surface ocean. The latter, termed the DFe replacement time, is around 1 year, which agrees well with recent estimates from comparable ocean regions. Key Points An imbalance between input and removal produces an ∼3‐fold seasonal increase in the euphotic‐zone inventory of dissolved iron (DFe) near Bermuda Analyses of iron in seasonal‐scale aerosol, rain and water‐column samples allow direct estimates of the replacement time of DFe We derive a mean residence time of ∼0.8–1.9 years for DFe in the euphotic zone (<200 m) of the Sargasso Sea near Bermuda

Empirical evidence in heavy fermion, pnictide and other systems suggests that unconventional superconductivity appears associated to some form of real-space electronic order. For the cuprates, despite several proposals, the emergence of order in the phase diagram between the commensurate antiferromagnetic state and the superconducting state is not well understood. Here we show that in this regime doped holes assemble in ‘electronic polymers’. Within a Monte Carlo study, we find that in clean systems by lowering the temperature the polymer melt condenses first in a smectic state and then in a Wigner crystal both with the addition of inversion symmetry breaking. Disorder blurs the positional order leaving a robust inversion symmetry breaking and a nematic order, accompanied by vector chiral spin order and with the persistence of a thermodynamic transition. Such electronic phases, whose properties are reminiscent of soft-matter physics, produce charge and spin responses in good accord with experiments. High-Tc superconductivity is thought to be associated with spatial electronic ordering, which for cuprates is not well understood yet. Here the authors use Monte Carlo simulations to show the emergence of a soft-matter-like electronic phase between the antiferromagnetic and the superconducting states.

Among the many intertwined phases in the cuprate superconductor phase diagram is the charge density wave (CDW) order, which has been detected in all major cuprate families. It is thought that CDW competes with superconductivity, but whether it has bearing on the mechanism of superconductivity remains unclear. Arpaia et al. undertook a comprehensive study of charge density fluctuations in a cuprate family, varying doping and temperature. They found that short-range dynamic charge fluctuations were present in a large portion of the phase diagram, at temperatures considerably higher than those at which the CDW order disappears. Science , this issue p. 906 Resonant inelastic x-ray scattering reveals dynamic charge fluctuations in a cuprate at high temperatures. Charge density modulations have been observed in all families of high–critical temperature ( T c ) superconducting cuprates. Although they are consistently found in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering, we carefully determined the temperature dependence of charge density modulations in YBa 2 Cu 3 O 7–δ and Nd 1+ x Ba 2– x Cu 3 O 7–δ for several doping levels. We isolated short-range dynamical charge density fluctuations in addition to the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T* , are characterized by energies of a few milli–electron volts, and pervade a large area of the phase diagram.

Inelastic Raman scattering is used to probe the critical spin fluctuations in an iron pnictide superconductor, providing insights into the origin of nematic order in this system. Nematic fluctuations and order play a prominent role in material classes such as the cuprates 1 , some ruthenates 2 or the iron-based compounds 3 , 4 , 5 , 6 and may be interrelated with superconductivity 7 , 8 , 9 , 10 , 11 . In iron-based compounds 12 signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin 13 , charge 9 , 14 or orbital 8 , 15 , 16 fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe 1− x Co x ) 2 As 2 (0 ≤ x ≤ 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths 17 , 18 , 19 , 20 , 21 . We show that the response from fluctuations appears only in B 1g ( x 2 − y 2 ) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at T s but vanishes only below the magnetic ordering transition at T SDW < T s , suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.

The goal of the Archimedes experiment is to investigate the role of the interaction between the vacuum fluctuations and gravitational field. This will be possible thanks to a high sensitivity and cryogenic balance installed in the SarGrav laboratory in the Sos Enattos mine (Sardinia), the Italian candidate site for the third generation gravitational wave observatory Einstein Telescope. Archimedes will measure the small weight variations induced in two high temperature superconductors that have the property of “trapping” or “expelling” vacuum energy when their temperatures are greater or lower than their critical temperatures. Only the radiative heat exchange mechanism must be used to remove or add thermal energy to the sample as it must be isolated from any external interaction that could add energy other than the vacuum one. The status of the experiment will be illustrated together with the most recent results.