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The recent report of near-ambient superconductivity and associated color changes in pressurized nitrogen doped lutetium hydride has triggered worldwide interest and raised major questions about the nature and underlying physics of these latest claims. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH2±xNy. We find that pressure conditions have notable effects on Lu-N and Lu-NH chemical bonding and the color changes likely stem from pressure-induced electron redistribution of nitrogen/vacancies and interaction with the LuH2 framework. No superconducting transition is found in all the phases at temperatures 1.8-300 K and pressures 0-38 GPa. Instead, we identify a notable temperature-induced resistance anomaly of electronic origin in LuH2±xNy, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of its novel pressure-induced phase changes. Near-ambient superconductivity and pressure-driven color changes were recently reported in nitrogen-doped lutetium hydride. Here, the authors synthesize LuH2±xNy and do not confirm the superconductivity. In addition, they find that the color changes likely stem from pressure-induced electron redistribution of nitrogen and vacancies. [ABSTRACT FROM AUTHOR]

A new method is developed for a more accurate computing of the value of the critical point for magnetic nanoparticles using the cellular automata simulation method. In traditional methods, the transition between positive and negative magnetization before the critical temperature results in erroneous computation of the critical point. Our method is based on the count of the states in the graph of magnetization versus time, in which the absolute value of magnetization per site is bigger than a threshold value mt. If the number of these states is more than 70% of the simulation time, the temperature of the system is below the Curie temperature; otherwise, the temperature is greater than the Curie temperature. According to the obtained results, it seems that this method is suitable for a better estimation of the value of the critical temperature. [ABSTRACT FROM AUTHOR]

We review the topological gauge theory of Josephson junction arrays and thin film superconductors, stressing the role of the usually forgotten quantum phase slips, and we derive their quantum phase structure. A quantum phase transition from a superconducting to the dual, superinsulating phase with infinite resistance (even at finite temperatures) is either direct or goes through an intermediate bosonic topological insulator phase, which is typically also called Bose metal. We show how, contrary to a widely held opinion, disorder is not relevant for the electric response in these quantum phases because excitations in the spectrum are either symmetry-protected or neutral due to confinement. The quantum phase transitions are driven only by the electric interaction growing ever stronger. First, this prevents Bose condensation, upon which out-of-condensate charges and vortices form a topological quantum state owing to mutual statistics interactions. Then, at even stronger couplings, an electric flux tube dual to Abrikosov vortices induces a linearly confining potential between charges, giving rise to superinsulation. [ABSTRACT FROM AUTHOR]

A series of FeSe0.5Te0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >104 A/cm2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor. [ABSTRACT FROM AUTHOR]

Under normal conditions, stoichiometric SrTiO3 is an excellent dielectric. It shows a structural phase transition, from cubic to tetragonal, below 105 K. In this structure, well separated domains hosting tetragonal phases with different long axes exist giving rise to the so-called X, Y, and Z domains. At very low temperatures, it becomes a quantum paraelectric in which local ferroelectric domains are found at the X, Y, and Z domain boundaries. Creation of oxygen vacancy in SrTiO3 makes it conducting with low carrier density which also undergoes an unconventional superconducting transition at sub-kelvin temperatures. We have created structural phase separation with clear domain boundaries (as in the X, Y, and Z domains) at room temperature on single crystals of SrTiO3 by irradiating the surface with high power density excimer laser pulses. We find that the domain boundaries are dominantly conducting, and the irradiated crystals undergo a superconducting phase transition below 180 mK indicating that the superconducting phase appears at the domain boundaries. This concurrence of local ferroelectricity and superconductivity in lightly doped SrTiO3 supports a ferroelectric fluctuation mediated Cooper pairing in the system. The results also point out the possibility of controlling ferroelectricity and superconductivity in functional electronic devices through surface engineering. [ABSTRACT FROM AUTHOR]

Superconducting high entropy alloys (HEAs) are a novel class of superconductors, with applications for electronic devices. Here, we investigated the effect of Mo alloying on superconducting properties of high entropy films with the composition (TaNb)1–x(ZrHfTi)xMoy. For near-equimolar composition, the crystalline HEAs grains are transformed into amorphous aggregations with a size in a few nanometer scale, forming a crystal/glass nanocomposite. In both crystalline and amorphous HEAs, the constituent atoms exhibit a homogeneous random distribution. The entropy-affected phase formations suppress the superconducting transitions in HEAs, which broadens the normal-to-superconducting transition regime and suppresses the zero-resistivity critical temperature to a lower constant value of approximately 2.9 K. [ABSTRACT FROM AUTHOR]

Under normal conditions, stoichiometric SrTiO3 is an excellent dielectric. It shows a structural phase transition, from cubic to tetragonal, below 105 K. In this structure, well separated domains hosting tetragonal phases with different long axes exist giving rise to the so-called X, Y, and Z domains. At very low temperatures, it becomes a quantum paraelectric in which local ferroelectric domains are found at the X, Y, and Z domain boundaries. Creation of oxygen vacancy in SrTiO3 makes it conducting with low carrier density which also undergoes an unconventional superconducting transition at sub-kelvin temperatures. We have created structural phase separation with clear domain boundaries (as in the X, Y, and Z domains) at room temperature on single crystals of SrTiO3 by irradiating the surface with high power density excimer laser pulses. We find that the domain boundaries are dominantly conducting, and the irradiated crystals undergo a superconducting phase transition below 180 mK indicating that the superconducting phase appears at the domain boundaries. This concurrence of local ferroelectricity and superconductivity in lightly doped SrTiO3 supports a ferroelectric fluctuation mediated Cooper pairing in the system. The results also point out the possibility of controlling ferroelectricity and superconductivity in functional electronic devices through surface engineering. [ABSTRACT FROM AUTHOR]

The superconducting state properties of the CuBa2Ca3Cu4O10+δ (Cu-1234) system, with a transition temperature as high as 117.5 K, were investigated. The ac magnetic susceptibility measurements confirmed a very sharp transition to the superconducting state. The upper critical field, Hc2, as high as 91 T, and the irreversibility field, Hirr, as high as 21 T at 77 K, were determined using dc SQUID magnetization measurements. The intragrain critical current density, jc, estimated from a magnetic hysteresis loop, is as high as 5 × 109 A/m2 in a self-generated magnetic field at 77 K. However, the intergrain critical current density in the studied material is smaller by four orders of magnitude due to very weak intergrain connections. [ABSTRACT FROM AUTHOR]

The article presents a study which fabricated Josephson Junctions of Kagome metal, K1-xV3Sb5 and induced superconductivity over long junction lengths. Topics discussed include proximity effect-induced superconductivity, magnetic field sweep direction dependence of mangetoresistance, and anisotropic interference pattern for in-plane and out-of-plane magnetic field.

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO2 planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba2Ca5Cu6O12(F,O)2 with inner CuO2 layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates. The phase diagram of cuprates is mostly based on data for single- and double-layered compounds in which the CuO2 planes are affected by the disorder. Here the authors report new features in the phase diagram of cuprates with low disorder by investigating the inner CuO2 planes of the six-layered compound. [ABSTRACT FROM AUTHOR]

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Most high- T c superconductors are spatially inhomogeneous. Usually, this heterogeneity originates from the interplay of various types of electronic ordering. It affects various superconducting properties, such as the transition temperature, the magnetic upper critical field, the critical current, etc. In this paper, we analyze the parameters of spatial phase segregation during the first-order transition between superconductivity (SC) and a charge- or spin-density wave state in quasi-one-dimensional metals with imperfect nesting, typical of organic superconductors. An external pressure or another driving parameter increases the transfer integrals in electron dispersion, which only slightly affects SC but violates the Fermi surface nesting and suppresses the density wave (DW). At a critical pressure P c , the transition from a DW to SC occurs. We estimate the characteristic size of superconducting islands during this phase transition in organic metals in two ways. Using the Ginzburg–Landau expansion, we analytically obtain a lower bound for the size of SC domains. To estimate a more specific interval of the possible size of the superconducting islands in (TMTSF) 2 PF 6 samples, we perform numerical calculations of the percolation probability via SC domains and compare the results with experimental resistivity data. This helps to develop a consistent microscopic description of SC spatial heterogeneity in various organic superconductors. [ABSTRACT FROM AUTHOR]

Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity. A promising candidate of the doped spin liquid is a triangular-lattice organic conductor, κ-(BEDT-TTF)4Hg2.89Br8, recently found to hold metallicity, spin-liquid-like magnetism, and BEC-like superconductivity. The nature of the metallic state with the spin-liquid behaviour is awaiting to be further clarified. Here, we report the thermoelectric signature that mobile holes in the spin liquid background are in a quantum critical state and it pertains to the BEC-like superconductivity. The Seebeck coefficient divided by temperature, S/T, is enhanced on cooling with logarithmic divergence indicative of quantum criticality. Furthermore, the logarithmic enhancement is correlated with the superconducting transition temperature under pressure variation, and the temperature and magnetic field profile of S/T upon the superconducting transition change with pressure in a consistent way with the previously suggested BEC-BCS crossover. The present results reveal that the quantum criticality in a doped spin liquid emerges in a phase, not at a point, and is involved in the unconventional BEC-like nature. A triangular-lattice organic conductor κ-(BEDT-TTF)4Hg2.89 Br8 is a promising doped spin liquid candidate which also exhibits superconductivity. Here the authors report thermoelectric measurements under pressure and find a quantum critical phase that could be correlated to BEC-like superconductivity. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

We have studied the influence of the piezoelectric properties of the PMN-PT substrate ([Pb(Mg1/3Nb2/3)O3]1-x – [PbTiO3]x where x = 0.29 − 0.31) on the properties of the superconducting spin valve Fe/Al/Co/Pb exposed to electric and magnetic fields. According to the research results, with an increase in the applied electric field to the PMN-PT substrate, it is possible to register an increase in the shift of the superconducting transition temperature of the superconducting spin valve Fe/Al/Co/Pb. The maximum shift was 36 mK when an electric field of 1 kV/cm was applied, which was half of the superconducting transition width. The shift of the superconducting transition temperature equal to 255 mK in a magnetic field was recorded, and thus the possibility of a full superconducting spin valve effect was shown. [ABSTRACT FROM AUTHOR]

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic susceptibilities. To demonstrate its operation, we detected superconducting phase transition in Nb and YBa2Cu3O7−δ, metal–insulator transitions in V2O3, ferromagnetic transition in Gd, and magnetic field induced transformation in meta magnetic NiCoMnIn single crystals. This high sensitivity apparatus allows the detection of trace amounts of materials (10−9-cc) undergoing an electromagnetic transition in a very broad temperature (2–400 K) and magnetic field (up to 90 kOe) ranges. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) are a branch of 2D materials which in addition to having an "easy‐to‐exfoliate" layered structure, also host semiconducting, metallic, superconducting, and topological properties in various polymorphs with potential applications. MoTe2 is an example of such a TMD, which shows semiconducting (in 2H phase), metallic (in 1T′ phase), topological Weyl semimetallic, and superconducting behavior (in Td phase). Consequently, an extensive amount of research has been done on MoTe2, particularly on the topological phase transition between the metallic‐type 1T′ phase and the topological Td phase. This phase transition has been reviewed and its association with the crystal structure, charge transport, and electronic band structure is elaborately discussed. Also, the effect of various stimuli like reduced dimensionality, pressure, charge doping, and chemical substitution, which affect the structural transition as well as the superconducting transition temperatures, is reviewed; thereby, suggesting certain correlations between the apparently unrelated structural and superconducting phase transitions. The review also brings out some open questions which are likely to interest the community to address the physics associated with the phase transition and its potential applications. [ABSTRACT FROM AUTHOR]

Lead-free BaTiO3 (BTO) film was fabricated by pulsed laser deposition on LaAlO3/SrTiO3 (LAO/STO) heterostructure in oxygen pressure of 2 × 10−3 Pa, which is experimentally required for ensuring the two-dimensional electron gas at LAO/STO heterointerface. Modulation of back-gate voltage on the superconductivity of LAO/STO and BTO/LAO/STO heterointerfaces was studied. The results indicate that the back-gate voltage can adjust the resistance at high temperatures around 300 mK, while it does not modulate the superconducting transition temperature of LAO/STO, which occurs at 160 mK. As for BTO/LAO/STO, it still shows a superconducting transition-like behavior at 110 mK. However, when a negative back-gate voltage is applied, the superconducting transition-like behavior disappears, and after removing the voltage, unlike what was observed for LAO/STO, the superconductivity cannot be recovered. Given this, some underlying mechanisms are proposed. [ABSTRACT FROM AUTHOR]

The EDELWEISS collaboration aims for direct detection of light dark matter using germanium cryogenic detectors with low threshold phonon sensor technologies and efficient charge readout designs. We describe here the development of Ge bolometers equipped with high impedance thermistors based on a NbxSi1−x TES alloy. High aspect ratio spiral designs allow the TES impedance to match with JFET or HEMT front-end amplifiers. We detail the behavior of the superconducting transition properties of these sensors and the detector optimization in terms of sensitivity to a-thermal phonons. We report preliminary results of a 200 g Ge detector that was calibrated using 71Ge activation by neutrons at the LSM underground laboratory. [ABSTRACT FROM AUTHOR]

Ultra-high purity elemental sources have long been considered a prerequisite for obtaining low impurity concentrations in compound semiconductors in the world of molecular beam epitaxy (MBE) since its inception in 1968. However, we demonstrate that a "dirty" solid precursor, ruthenium(III) acetylacetonate [also known as Ru(acac)3], can yield single-phase, epitaxial, and superconducting Sr2RuO4 films with the same ease and control as III–V MBE. A superconducting transition was observed at ∼0.9 K, suggesting a low defect density and a high degree of crystallinity in these films. In contrast to the conventional MBE, which employs the ultra-pure Ru metal evaporated at ∼2000 °C as a Ru source, along with reactive ozone to obtain Ru → Ru4+ oxidation, the use of the Ru(acac)3 precursor significantly simplifies the MBE process by lowering the temperature for Ru sublimation (less than 200 °C) and by eliminating the need for ozone. Combining these results with the recent developments in hybrid MBE, we argue that leveraging the precursor chemistry will be necessary to realize next-generation breakthroughs in the synthesis of atomically precise quantum materials. [ABSTRACT FROM AUTHOR]

The design of epitaxial interfaces is a pivotal way to engineer artificial structures where new electronic phases can emerge. Here, we report a systematic emergence of an interfacial superconducting state in epitaxial heterostructures of LaTiO3 and KTaO3. The superconductivity transition temperature increases with decreasing thickness of LaTiO3. Such a behavior is observed for both (110) and (111) crystal oriented structures. For thick samples, the finite resistance developing below the superconducting transition temperature increases with increasing LaTiO3 thickness. Consistent with previous reports, the (001) oriented heterointerface features a high electron mobility of 250 cm2 V−1 s−1 and shows no superconducting transition down to 40 mK. Our results imply a non-trivial impact of LaTiO3 on the superconducting state and indicate how superconducting KTaO3 interfaces can be integrated with other oxide materials. [ABSTRACT FROM AUTHOR]

This article reports the effect of a nanoscale addition of TiO2 on the structure and superconducting parameters of the high-temperature superconductor YBa2Cu3O7-δ (Y123). Polycrystalline compounds of Y123 with different percentages of TiO2, x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, were fabricated using the thermal treatment method. An analysis using X-ray diffraction confirmed the formation of Y123 phases for all composite samples. Field-emission scanning electron microscopy (FESEM) analysis revealed the growth of grain size and decrease in porosity, with a sign of partial melting of grains for the samples with TiO2 addition. The magnetic and electric transport properties were investigated using AC susceptibility measurement and the four-probe method, respectively. It was observed that the superconducting transition temperature, Tc-onset, for a pure sample determined by ACS and 4PP was 95.6 K and 95.4 K, respectively. These values were found to decrease with the addition of TiO2, while the superconducting transition (∆Tc) improved with TiO2 addition except for the sample at x = 0.2 wt.%, which showed the broadest transition width. The sharpest superconducting transition (∆Tc) was observed for the sample at x = 1.0 wt.%, indicating that the addition of TiO2 nanoparticles is expected to serve as artificial pinning centres and strengthen the connection among the grains in the Y123 ceramic. [ABSTRACT FROM AUTHOR]

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca10(Pt4As8)((Fe0.97Pt0.03)2As2)5 superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A□) gradually diminishes. At a critical pressure, A□ and superconducting transition temperature (Tc) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A□ with Tc obeys the relation of Tc ~ (A□)0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-Tc superconductivity. The strange metal state, characterized by a linear-in-temperature resistivity, is often seen in the normal state of high-temperature superconductors. Here, the authors report the breakdown of both the strange metal and superconducting states at a pressure-induced quantum critical point in an iron-pnictide superconductor. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

We report structural and superconducting transitions in layered van der Waals (vdW) palladium dihydride (PdH 2) calculated under high-pressure compression. PdH 2 has a Hexagonal Closed-Packed (HCP) structure with a space group of P6 3 mc , and has a superconducting transition temperature of 24 K at ambient pressure. At 15 GPa, the crystalline to vdW layered structural transition occurs, while the superconductivity remains. On compressing from 15 to 50 GPa, the T c increased abnormally by 3.5 K. It is found that the superconducting critical temperature of P6 3 mc PdH 2 is determined by the out-of-plane interlayer breathing vibrational mode. As a vdW layered metal hydride superconductor, PdH 2 provides a platform to study hydride superconductivity in such kinds of materials. • PdH 2 has a superconducting transition temperature of 24 K at ambient pressure. • The crystalline to vdW layered structural transition occurs at 15 GPa. • The abnormal high-T c at high pressure is associated with the out-of-plane interlayer breathing vibrational mode in our model. [ABSTRACT FROM AUTHOR]

Unconventional superconductivity typically refers to a superconducting state that stems from an effective attractive interaction between electronic quasiparticles, which is not the canonical electron-phonon coupling (EPC) [[1]]. IXS measurements have recently shown that all signatures of the CDW in the low-energy phonon spectra of this compound, such as the mode broadening above I T SB c sb i or the phonon softening, were suppressed with a relatively modest pressure of ~1 GPa (Figures 2 (a) and (b)) [[31]]. Compared to oxygen bond-stretching phonons in cuprates, the relevant ions are heavy and, thus, make IXS much more favorable for phonon spectroscopy in iron-based superconductors. Iron-based superconductors: Phonons and nematic fluctuations Superconductivity in many iron-based superconductors emerges when an antiferromagnetic spin-density-wave (SDW) ground state is suppressed either by doping or pressure [[68]] where the magnetic ordering is accompanied or preceded by a transition from a tetragonal ( I C i SB 4 sb ) to orthorhombic ( I C i SB 2 sb ) structure [[69], [71]]. [Extracted from the article]

We will consider first the orbital symmetry of CDW order at finite Graph HT ht . More precisely, for atoms, I j i , in the lattice having the same electronic structure, Graph HT ht is a complex quantity, with the imaginary part Graph HT ht proportional the atomic absorption cross-section of atoms Graph HT ht ; Graph HT ht . [Extracted from the article]

Quantum simulation enables study of many-body systems in non-equilibrium by mapping to a controllable quantum system, providing a powerful tool for computational intractable problems. Here, using a programmable quantum processor with a chain of 10 superconducting qubits interacted through tunable couplers, we simulate the one-dimensional generalized Aubry-André-Harper model for three different phases, i.e., extended, localized and critical phases. The properties of phase transitions and many-body dynamics are studied in the presence of quasi-periodic modulations for both off-diagonal hopping coefficients and on-site potentials of the model controlled respectively by adjusting strength of couplings and qubit frequencies. We observe the spin transport for initial single- and multi-excitation states in different phases, and characterize phase transitions by experimentally measuring dynamics of participation entropies. Our experimental results demonstrate that the recently developed tunable coupling architecture of superconducting processor extends greatly the simulation realms for a wide variety of Hamiltonians, and can be used to study various quantum and topological phenomena. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

The successful integration of superconducting niobium (Nb) gate electrodes into cryogenic gallium nitride (GaN)‐based high‐electron‐mobility transistors (HEMTs) is reported. This is achieved through a specifically developed microfabrication process. The device's DC, microwave, and noise performances at cryogenic temperatures, down to 4 K, are studied and presented. The superconductivity of the gate is tested using DC end‐to‐end measurements. A clear superconducting state transition at a critical temperature, Tc, of ≈9.2 K is shown. This is further verified with GaN HEMTs with two gate fingers and a gate length of 0.2 μm, through the extraction and validation of a small‐signal model at T < Tc. Additionally, the superconductivity of the gate is verified for several gate widths and lengths, showing a significant reduction of the gate resistance independently of its dimensions. Finally, a comparative study of the cryogenic microwave noise performances of the GaN HEMTs with gold (Au) and Nb gates is presented. The Au‐gated device presents a competitive optimum noise temperature, Tmin‐opt, of ≈8 K at 5 GHz, demonstrating the potential of this technology for cryogenic low‐noise applications. The Nb‐gated device presents a 5 K higher Tmin‐opt, which is found to be related to the suppression of the superconductivity of the Nb gate at the optimum‐noise bias. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

The enhanced performance of superconducting FeSe0.5Te0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe0.5Te0.5 (FeSe0.5Te0.5 + xPb + ySn; x = y = 0–0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (Tconset) of pure FeSe0.5Te0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce Tconset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe0.5Te0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, Jc, and the pinning force, Fp, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity. [ABSTRACT FROM AUTHOR]

We have investigated the correlation between structural and transport properties in sputtered β -FeSe films grown onto SrTiO 3 (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced T c ∼ 12 K, a relatively high H c 2 , and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed to subtle details of the multiband electronic structure of β -FeSe. Regarding the superconductor-insulator transition induced by reducing the thickness, we found a nontrivial evolution of the structural properties and morphology associated with a strained initial growth and the coalescence of grains. Finally, we discuss the origin of the insulating behavior in high-quality stressed epitaxial thin films. We found that a lattice distortion, described by Poisson's coefficient associated with the lattice parameters a and c, may play a key role. [ABSTRACT FROM AUTHOR]

Small changes in electrical conductance of quasi-2 dimensional samples often need to be measured at low temperatures, T ∼ 1 K and lower. Care needs to be taken to minimize self-heating due to the excitation current itself and the possibility of any unexpected source of variation of the electrochemical potential around the contacts need to be understood. It is not only the low resistance of the ohmic contacts but also their "noninvasive" character that must be ensured. While the importance of the "noninvasiveness" of the voltage probes is well appreciated for mesoscopic devices—it turns out that these considerations continue to remain important even in macroscopic samples that are a few millimeters in size. We have done low temperature measurements on different types of gallium nitride samples with contacts made of pure indium (popularly called indium dots), which have a superconducting transition and contacts made of Ti/Au, which remain nonsuperconducting. We show that there are qualitative differences in the measured four-terminal resistance in the two cases. Obviously, correct conclusions about phase relaxation times, localization lengths, etc., will require ensuring that there are no effects arising from the "invasiveness" of the probes. [ABSTRACT FROM AUTHOR]

In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance. [ABSTRACT FROM AUTHOR]

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV3Sb5, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV3Sb5. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV3Sb5. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV3Sb5. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V0.93Nb0.07)3Sb5. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV3Sb5. Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV3Sb5, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV3Sb5 is strong enough to support the experimental superconducting transition. [ABSTRACT FROM AUTHOR]

One-state and two-state lasing is investigated experimentally and through numerical simulation as a function of temperature in microdisk lasers with Stranski–Krastanow InAs/InGaAs/GaAs quantum dots. Near room temperature, the temperature-induced increment of the ground-state threshold current density is relatively weak and can be described by a characteristic temperature of about 150 K. At elevated temperatures, a faster (super-exponential) increase in the threshold current density is observed. Meanwhile, the current density corresponding to the onset of two-state lasing was found to decrease with increasing temperature, so that the interval of current density of pure one-state lasing becomes narrower with the temperature increase. Above a certain critical temperature, ground-state lasing completely disappears. This critical temperature drops from 107 to 37 °C as the microdisk diameter decreases from 28 to 20 μm. In microdisks with a diameter of 9 μm, a temperature-induced jump in the lasing wavelength from the first excited-state to second excited-state optical transition is observed. A model describing the system of rate equations and free carrier absorption dependent on the reservoir population provides a satisfactory agreement with experimental results. The temperature and threshold current corresponding to the quenching of ground-state lasing can be well approximated by linear functions of saturated gain and output loss. [ABSTRACT FROM AUTHOR]

Using the RT formula and the subregion CV conjecture, we numerically investigate the holographic entanglement entropy (HEE) and holographic subregion complexity (HSC) for two holographic d-wave superconducting models with backreactions. We find that the HEE and HSC can probe these two d-wave superconducting phase transitions. The HEE of the superconducting phase is always lower than that of the normal phase. For the HSC, however, it behaves differently and interestingly, which depends on both the strip-width L x and backreaction κ . More specifically, when the backreaction is larger than a particular critical value, or the strip-width of the boundary subsystem is smaller than a particular critical value, the HSC in the superconducting phase is larger than that in the normal phase, and vice versa. [ABSTRACT FROM AUTHOR]

For the conventional superconductors it will be shown that not only the superconducting energy gap, Egap(T=0), and the critical field, Bc(T=0), but also the London penetration depth, λL(T=0), scale in a reasonable approximation with the superconducting transition temperature, TSC, as ~TSC, ~TSC 2 and ~T-1/2, respectively. From these scaling relations the conclusion obtained earlier, using a completely different method, is confirmed that the London penetration depth corresponds to the diameter of the Cooper-pairs. As a consequence, only one layer of Cooper pairs is sufficient to shield an external magnetic field completely. The large diamagnetism of the superconductors is caused by the large orbital area of the Cooper-pairs. From the fact that, in the zero-field ground state, the temperature dependence of the superconducting heat capacity is given above and below TSC by power functions of absolute temperature it follows that the only critical point is T=0. The superconducting transitions of the element superconductors, therefore, are all within the critical range at T=0. As a consequence, above and below TSC there is short-range order only. As we know from Renormalization Group (RG) theory, in the critical range the dynamics is the dynamics of a boson field, exclusively. Evidently, the Cooper-pairs have to be considered as the short-range ordered units created by this boson field. It is reasonable to assume that the relevant bosons in the superconducting state are identical with the bosons giving rise to the universal linear-in-T electronic heat capacity above TSC. Plausibility arguments will be given that these bosons must be electric quadrupole radiation generated by the non-spherical charge distributions in the soft zones between the metal atoms. The radiation field emitted by an electric quadrupole can be assumed to be essentially curled or circular. In the ordered state below TSC, the bosons are condensed in resonating spherical modes which encapsulate the two Cooper-pair electrons and shield their charge perfectly. [ABSTRACT FROM AUTHOR]

The domains and domain boundaries in 2D materials are known to play essential roles in investigating intriguing physical properties and have potential applications in nanoscale devices. Understanding the influence of individual domains on the superconducting properties of ultrathin 2D superconductors is of crucial importance for fundamental studies on mesoscopic superconductivity as well as applications in superconducting nanoelectronics. Here, low‐temperature electronic transport measurements of high quality ultrathin Mo2C crystals are presented that show clear evidence for the presence of multiple superconducting phase induced by the nanoscale domain structures. In particular, the observation of an anomalous resistance peak in the vicinity of the onset of the superconducting transition is reported. This resistive anomaly is interpreted as a consequence of nonequilibrium charge imbalance near the domain boundaries, which could induce effective normal‐superconducting interfaces in 2D Mo2C crystals. Moreover, the magnetic field‐tuned superconductor‐metal transition for ultrathin Mo2C crystals is examined. The observed scaling behavior is consistent with the appearance of quantum Griffiths singularity in 2D superconducting systems. This study sheds light on the understanding of the domain boundaries and their role on the transport properties of highly crystalline 2D superconductors, which may open potential application of domain structure in functional superconducting nanodevices. [ABSTRACT FROM AUTHOR]

We have tried to grow α-Sn films on two different substrates with different sample structures by molecular beam epitaxy. The mixture of an α phase with a β phase in the Sn film has been confirmed. The electrical transport properties have been measured and multiple superconducting transitions have been observed in these α-Sn/β-Sn mixed films. Enhanced critical temperatures and critical fields as well as shorter coherence lengths are observed, as compared to bulk β-Sn. The two-dimensional nature is demonstrated by angle-dependent measurements and a type-II superconductor is indicated by the magnetization measurements. We attribute the multiple superconducting transitions to the β-Sn islands formed within the film. [ABSTRACT FROM AUTHOR]

This paper describes a low-noise microwave amplifier based on up- and down-frequency-conversion processes in quasiparticle superconductor–insulator–superconductor (SIS) tunnel junctions. The SIS amplifier was configured with two SIS frequency-converter modules and a cryogenic millimeter-wave isolator inserted between them. Moreover, a local oscillator (LO) using millimeter-wave attenuators and a phase shifter was considered. This setup allowed the control of individual LO power and differential phase in these SIS frequency converters to optimize the amplifier performance. The SIS amplifier showed noise temperatures as low as 11 K and a 6–8 dB gain from nearly DC to 5 GHz. The attained microwave performance is promising for obtaining large-format arrays, such as multibeam heterodyne receivers. Moreover, this two-frequency-converter concept based on SIS junctions might enable microwave applications, such as wideband non-reciprocal circuits in isolators, gyrators, and circulators, which are essential devices in the quantum computing and radio astronomy fields. [ABSTRACT FROM AUTHOR]

We review the topological gauge theory of the superconductor-to-superinsulator transition. The possible intermediate Bose metal phase intervening between these two states is a bosonic topological insulator. We point out that the correct treatment of a bosonic topological insulator requires a normally neglected, additional dimensionless parameter, which arises because of the non-commutativity between the infinite gap limit and phase space reduction. We show that the bosonic topological insulator is a functional first Landau level. The additional parameter drives two Berezinskii–Kosterlitz–Thouless (BKT) quantum transitions to superconducting and superinsulating phases, respectively. The two BKT correlation scales account for the emergent granularity observed around the transition. Finally, we derive the ground state wave function for a system of charges and vortices in the Bose metal phase. [ABSTRACT FROM AUTHOR]

The photo-induced superconducting phase transition is widely used in probing the physical properties of correlated electronic systems and to realize broadband photodetection with extremely high responsivity. However, such photoresponse is usually insensitive to electrostatic doping due to the high carrier density of the superconductor, restricting its applications in tunable optoelectronic devices. In this work, we demonstrate the gate voltage modulation to the photoresponsivity in a two-dimensional NbSe2-graphene heterojunction. The superconducting critical current of the NbSe2 relies on the gate-dependent hot carrier generation in graphene via the Joule heating effect, leading to the observed shift of both the magnitude and peak position of the photoresponsivity spectra as the gate voltage changes. This heating effect is further confirmed by the temperature and laser-power-dependent characterization of the photoresponse. In addition, we investigate the spatially-resolved photocurrent, finding that the superconductivity is inhomogeneous across the junction area. Our results provide a new platform for designing tunable superconducting photodetector and indicate that the photoresponse could be a powerful tool in studying the local electronic properties and phase transitions in low-dimensional superconducting systems. [ABSTRACT FROM AUTHOR]

Ferromagnetic–insulator–superconductor Co2CrAl–I–Pb tunnel junctions were created and their current-voltage characteristics (CVCs) were studied. It has been established that they differ from the usual CVCs of tunnel junctions of a normal metal–insulator–superconductor. It is proposed to assume that the observed effect of changing the type of CVC is associated with the existence of the destruction of Cooper pairs due to the inverse effect of proximity between the Co2CrAl and Pb electrodes. The CVCs were calculated based on the approach of P. Fulde. [ABSTRACT FROM AUTHOR]

The magnetization induced in a superconductor due to the reverse proximity effect is studied in hybrid structures containing a superconductor and a ferromagnetic insulator. The study was carried out within the method of semiclassical Green's functions, in which the Usadel equations are solved numerically with boundary conditions suitable for strongly spin-polarized ferromagnetic materials. The conversion of singlet superconducting correlations into triplet ones as a result of the proximity effect with a ferromagnet and its manifestation in the features of the electron density of states, induced magnetization, and suppression of the superconducting order parameter have been studied. It is shown that the magnetization can change sign inside the superconducting layer. The magnetization distribution is compared with the data obtained by the authors in previous works. [ABSTRACT FROM AUTHOR]

For applications of nanolattices in low-temperature nanoelectronics, the inter-unit space can be filled with superconducting metallic alloys. However, superconductivity under nanoconfinement is expected to be strongly affected by size-effects and other factors. We studied the magnetic properties and structure of the Ga-Sn eutectic alloy within regular nanopores of an opal template, to understand the specifics of the alloy superconductivity. Two superconducting transitions were observed, in contrast to the bulk alloy. The transitions were ascribed to the segregates with the structures of tetragonal tin and a particular gallium polymorph. The superconducting-phase diagram was constructed, which demonstrated crossovers from the positive- to the common negative-curvature of the upper critical-field lines. Hysteresis was found between the susceptibilities obtained at cooling and warming in the applied magnetic field. [ABSTRACT FROM AUTHOR]

The fabrication of a Fe-based coated conductor (CC) becomes possible when Fe(Se,Te) is grown as an epitaxial film on a metallic oriented substrate. Thanks to the material's low structural anisotropy, less strict requirements on the template microstructure allow for the design of a simplified CC architecture with respect to the REBCO multi-layered layout. This design, though, still requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, Fe(Se,Te) films are grown on chemically-deposited, CeO2-based buffer layers via pulsed laser deposition, and excellent properties are obtained when a Fe(Se,Te) seed layer is used. Among all the employed characterization techniques, transmission electron microscopy proved essential to determine the actual effect of the seed layer on the final film properties. Also, systematic investigation of the full current transport properties J(θ, H, T) is carried out: Fe(Se,Te) samples are obtained with sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm−2 at 4.2 K in self-field. The in-field and angular behavior of the sample are in line with data from the literature. These results are the demonstration of the feasibility of a Fe-based CC, with all the relative advantages concerning process simplification and cost reduction. [ABSTRACT FROM AUTHOR]

Unconventional superconductors often feature competing orders, small superfluid density, and nodal electronic pairing. While unusual superconductivity has been proposed in the kagome metals AV3Sb5, key spectroscopic evidence has remained elusive. Here we utilize pressure-tuned and ultra-low temperature muon spin spectroscopy to uncover the unconventional nature of superconductivity in RbV3Sb5 and KV3Sb5. At ambient pressure, we observed time-reversal symmetry breaking charge order below T 1 * ≃ 110 K in RbV3Sb5 with an additional transition at T 2 * ≃ 50 K. Remarkably, the superconducting state displays a nodal energy gap and a reduced superfluid density, which can be attributed to the competition with the charge order. Upon applying pressure, the charge-order transitions are suppressed, the superfluid density increases, and the superconducting state progressively evolves from nodal to nodeless. Once optimal superconductivity is achieved, we find a superconducting pairing state that is not only fully gapped, but also spontaneously breaks time-reversal symmetry. Our results point to unprecedented tunable nodal kagome superconductivity competing with time-reversal symmetry-breaking charge order and offer unique insights into the nature of the pairing state. The nature of the superconductivity in the kagome metals AV3Sb5 (A = K, Rb, Cs) remains under debate. Here, using muon spin spectroscopy, the authors find that the superconductivity in RbV3Sb5 and KV3Sb5 evolves from nodal to nodeless with increasing pressure and the superconducting state breaks time-reversal symmetry after suppression of the charge order. [ABSTRACT FROM AUTHOR]