Superconducting Phase Research Articles (Page 1)

Among the various mysteries in cuprate high temperature superconductors, the pseudogap (PG) phase stands out for the difficulty in pinning down its origin and its close connection to unconventional superconductivity. It appears above the superconducting transition temperature ​, where part of the low energy spectral weight becomes depleted and several competing or intertwined orders such as charge and pair density waves, nematicity, and spin fluctuations begin to develop. A persistent challenge lies in the systematic discrepancies revealed by different experimental probes, as transport measurements locate the critical doping near , spectroscopic studies around , and symmetry sensitive techniques close to . These variations reflect the distinct sensitivities of each probe to correlation length scales and electronic coherence rather than experimental inconsistency. This review brings together evidence from angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy or spectroscopy (STM or STS), nuclear magnetic resonance (NMR), resonant X ray scattering (RXS), and optical conductivity, showing that the pseudogap is a spatially heterogeneous, symmetry breaking electronic state whose onset temperature decreases roughly linearly with doping and terminates sharply at . Three complementary theoretical frameworks, namely quantum criticality, Mott physics, and intertwined orders, collectively describe these observations. Experiments showing the abrupt disappearance of nematic order and a logarithmic rise in the electronic specific heat coefficient suggest that the pseudogap terminates at a quantum critical point. This transition appears to separate a correlation dominated pseudogapped metal from a coherent Fermi liquid phase rather than occurring through a gradual crossover. The doping level identified from transport data aligns with optimal superconductivity, implying that the recovery of long range phase coherence rather than the complete removal of pseudogap features is what ultimately enhances . Unresolved questions include reconciling probe dependent boundaries through systematic cross technique studies on identical crystals and developing correlation length resolved probes to distinguish spatial scales of electronic reconstruction. A major theoretical challenge remains to unify competing frameworks and to elucidate how the pseudogap terminates and coherence emerges at , which represents a key step toward a microscopic theory of high temperature superconductivity in cuprates.

Metal intercalation in epitaxial graphene enables the emergence of proximity-induced superconductivity and modified quantum transport properties. However, systematic transport studies of intercalated graphene have been hindered by challenges in device fabrication, including processing-induced deintercalation and instability under standard lithographic techniques. Here, a lithographically controlled intercalation approach is introduced that enables the scalable fabrication of gallium-intercalated quasi-freestanding bilayer graphene (QFBLG) Hall bar devices. By integrating lithographic structuring with subsequent intercalation through dedicated intercalation channels, this method ensures precise control over metal incorporation while preserving device integrity. Magnetotransport measurements reveal superconductivity with a critical temperature ≈ 3.5 K and the occurrence of a transverse resistance, including both symmetric and antisymmetric field components, which is attributed to the symmetric-in-field component of non-uniform currents. These results establish an advanced fabrication method for intercalated graphene devices, providing access to systematic investigations of confined 2D superconductivity and emergent electronic phases in van der Waals heterostructures.

1T-TaS2 is a prototype layered material with a rich phase diagram that includes multiple charge density wave (CDW) transitions and technologically important metastable states. It also supports a superconducting phase induced by hydrostatic pressure, cation substitution, intercalation, or doping. Thin 1T-TaS2 crystals deposited on various substrates exhibit transition temperatures that are strongly dependent on the substrate-induced strain, and depart from bulk transition temperatures in a way that is not clearly understood at present. Here we show that thin polycrystalline films of 1T-TaS2 grown by molecular beam epitaxy on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) substrates have a suppressed CDW transition to a commensurate phase. Instead, resistivity, magnetoresistance, and critical current measurements reveal metallic behavior with an onset to a superconducting state below [Formula: see text] K. The appearance of superconductivity is suggested to be driven by the in-plane tensile differential strain exerted on the 1T-TaS2 film by the LSAT substrate during cooling, which in turn results in a strongly amplified out-of-plane compressive strain triggered by the Poisson effect, combined with traceable signs of intercalation with La and Sr atoms from the substrate. The experiments suggest that tensile substrate strain may be usefully applied for achieving desirable Functional properties that are otherwise accessible through hydrostatic pressure, and generally for investigating of the effects of anisotropic strain in 2D materials and monolayer stacks or heterostructures.

Two-dimensional magnet-superconductor hybrids (2D-MSH) are promising candidates to realize devices for topology-based quantum technologies and superconducting spintronics. So far, studies have focused on 2D-MSH systems with collinear ferro- or antiferromagnetic layers. Here, we present the discovery of topological superconductivity in a noncollinear MSH system where a magnetic spiral is realized in an Fe monolayer proximity coupled to a superconducting Ta(110) substrate. By combining low-temperature spin-polarized scanning tunneling spectroscopy with an in-depth theoretical study, we can conclude that the system is in a topological nodal-point superconducting phase with low-energy edge modes. Furthermore, we reveal that for this noncollinear spin texture, these edge modes exhibit a magnetization direction-dependent dispersion. This means that a spatial shift of the magnetic spiral could be used to reverse the chirality of an edge mode in future MSH-based devices.

Exploring emergent phases in monolayer alloy superconductors represents a forefront endeavor in contemporary quantum materials research. Following the successful exploration of AlB2 in a superconducting state, we provide a significant reference for examining superconductivity in Si-substituted AlB2 using first-principles predictions. This noteworthy outcome highlights that Al0.75Si0.25B2 is one of the energetically stable configurations within the Al1-xSixB2 system that exhibits the superconducting state. However, the anharmonic effects on this phase significantly impact its phonon spectra, potentially influencing dynamical stability. In specific cases, the application of the stochastic self-consistent harmonic approximation enables us to capture how thermally induced lattice vibrations impact the equilibrium structure of the material. It is observed that the inclusion of anharmonic corrections brings the predicted superconducting characteristics into closer agreement with those derived from the harmonic model, thereby resolving the issue of imaginary frequencies. As a result, we demonstrate that the Allen-Dynes modified McMillan scheme predicts a critical temperature (Tc) of approximately 15 K. This can be enhanced to 41 K through the utilization of the anisotropic Migdal-Eliashberg theory. Our findings reveal that the role of anharmonicity-arising from minor corrections in the acoustic regime contributed by the high atomic mass-in Al0.75Si0.25B2 theoretically leads to superconductivity, with Tc being consistent with values predicted within the harmonic approximation.

Phonons play a central role in fundamental solid-state phenomena, including superconductivity, Raman scattering, and symmetry-breaking phases. Harnessing phonons to control these effects and enable quantum technologies is therefore of great interest. However, most existing phonon control strategies rely on external driving fields or anharmonic interactions, limiting their applicability. Here, we realize multimode ultrastrong light-matter coupling and theoretically show the modulation of phonon emission. This regime is realized by coupling two optical phonon modes in lead halide perovskites to a nanoslot array functioning as a single-mode cavity. The small mode volume of the nanoslots enables high coupling strengths in the phonon-polariton system. We show theoretically that the nanoslot resonator mediates an effective interaction between phonon modes, leading to superthermal phonon bunching in thermal equilibrium between distinct modes. Our findings are well described by a multimodal Hopfield model. This work establishes a pathway for engineering phononic properties for light-harvesting and light-emitting technologies.

Quasi-one-dimensional systems have garnered significant attention owing to the exotic properties they can host including superconductivity, charge density waves, topological spin excitations and more. Pressure-induced superconductivity has been realized in a new family of Mn-based Q1D materials, AMn6Bi5 (A = K, Rb, Cs, Na), with unique [Mn6Bi5]−1 double-walled columns. The smallest countercation Na+ yields the highest chemical pressure experienced in this family, reducing the Mn interatomic bond lengths and enhancing the metallicity and magnetic frustration within the Mn pentagonal antiprisms, thus, driving NaMn6Bi5 closer to the high-pressure superconducting phase. Distinct from the single magnetic transition in other family members, NaMn6Bi5 goes through multiple magnetic transitions at TN1 ∼88 K, TN2 ∼52 K and TN3 ∼48 K. In this talk I will present the findings of the unique low temperature, below TN3 ∼48 K, noncolinear “all-in-all-out” pentagonal antiferromagnetic order and high temperature in-plane moment dispersed pentagon phase in NaMn6Bi5 determined from single crystal neutron diffraction. The low temperature “all-in-all-out” state exhibits spins pointing all towards or away from the center of the pentagon and alternating down the Mn pentagonal antiprism columns along the b axis. The innermost central Mn-site continuously shows no/negligible ordered moment, resulting from the magnetic frustration within the Mn pentagonal antiprisms and nearly metallic bond distances. High pressure X-ray diffraction up to 18.5 GPa revealed no additional lattice transition, indicating the magnetic variation under pressure is highly relevant to the high-pressure superconducting phase found in this family. This investigation has, therefore, shed new light on the rare one-dimensional Mn-based superconductors.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an unusual superconducting phase that survives beyond the Pauli paramagnetic limit through spatial modulation of the order parameter. An even more exotic variant—the orbital FFLO state—was recently reported in thin flakes of 2H-NbSe2, involving the interplay of Ising spin-orbit coupling and orbital pair breaking. Here, we report thermodynamic signatures consistent with an orbital FFLO state in bulk 2H-NbSe2, based on high-resolution magnetization and torque measurements under strictly parallel to the NbSe2 basal plane. In the magnetic phase diagram, a crossover to a first-order transition appears above 3 T and disappears with slight field misalignment, indicating field-angle dependent Pauli-limited behavior. Additionally, we observe a reversible step-like anomaly within the superconducting state, and a pronounced six-fold in-plane modulation of the upper critical field above this phase transition. These results suggest that the orbital FFLO state is likely realized even in the bulk limit of 2H-NbSe2.

The heavy-fermion superconductor (SC) CeRh2As2 (Tc = 0.35 K) shows two SC phases SC1 and SC2 when a magnetic field is applied parallel to the c axis of the tetragonal unit cell. All experiments to date have indicated that the change in SC order parameter detected at μ0H*≈4T is due to strong Rashba spin-orbit coupling at the Ce sites caused by the locally noncentrosymmetric environments of the otherwise globally centrosymmetric crystalline structure. Another phase (phase I) exists in this material below T0 = 0.54 K. In a previous specific heat study [K. Semeniuk , ], we have shown that phase I persists up to a field μ0H0≈6T, larger than H*. From thermodynamic arguments, we expected the phase-I boundary line to cross phase SC2 at a tetracritical point. However, we could not find any signature of the phase-I line inside the SC2 phase and speculated that this was because the T0(H) line is almost perpendicular to the H axis and therefore invisible to T-dependent measurements. This would imply a weak competition between the two order parameters. Here, we report magnetic-field-dependent measurements of the magnetostriction and ac susceptibility on high-quality single crystals. We see clear evidence of the singularity at H0 inside the SC2 phase and confirm our previous prediction. Furthermore, we observe the transition across the T*(H) line in T-dependent specific heat measurements, which show that the T*(H) line is not perpendicular to the field axis but has a positive slope. Our findings support recent muon spin resonance results which suggest coexistence of phase I with SC.

Superconducting phase interference effect in momentum space.

Abstract In a superconductor embedded with a quantum magnetic impurity, the Kondo effect is involved, leading to the competition between the Kondo singlet phase and the superconductivity phase. By means of the natural orbitals renormalization group (NORG) method, we revisit the problem of a quantum magnetic impurity coupled with a conventional s-wave superconductor. Here we present a detailed study focusing on the impurity spin polarization and susceptibility, the Kondo screening cloud, as well as the number and structures of the active natural orbitals (ANOs). In the superconducting phase, the impurity spin is partially polarized, indicating that the impurity remains partially screened by the quantum fluctuations. Furthermore, the impurity spin susceptibility becomes divergent, resulting from the presence of residual local moment formed at the impurity site. Correspondingly, a non-integral (incomplete) Kondo cloud is formed, although the ground state is a spin doublet in this phase. In comparison, the Kondo cloud is complete in the Kondo singlet phase as expected. We also quantify the critical point, where the quantum phase transition from a Kondo singlet phase to a superconducting phase occurs, which is consistent with that in previous works. On the other hand, it is illustrated that only one ANO emerges in both quantum phases. The structures of the ANO, projected into both the real space and momentum space, are distinct in the Kondo singlet phase from that in the superconducting phase. More specifically, in the Kondo singlet phase, the ANO keeps fully active with half-occupied, and the superconducting gap has negligible influence on its structure. On the contrary, in the superconducting phase, the ANO tends to be inactive and its structure changes significantly as the superconducting gap increases. Additionally, our investigation demonstrates that the NORG method is reliable and convenient to solve the quantum impurity problems in superconductors as well, which will promote further theoretical studies on the Kondo problems in such systems using numerical methods.

In the last years, kagome materials have received massive attention by virtue of being candidate hosts for a large variety of quantum phases: spin liquids, unconventional superconductivity, and topological phases of matter, to name the more exotic. One of the most interesting features is tunability: changing the filling, the noninteracting band structure can be tuned from flat bands to conventional metallic phases as well as to semimetals. In this paper, we concentrate on the latter. At specific lattice filling, the electronic bands have a semimetallic structure, hosting Dirac, massless quasiparticles, such as in graphene or other layered two dimensional materials. Specifically, we determine what terms can be added to the nearest-neighbor hopping that opens at the gap at the said Dirac point. These terms can, in principle, arise through external perturbations, interactions, or collective instabilities. We classify the 16 possible gap-opening terms according to the broken symmetries. Furthermore, we identify concrete microscopic realizations, allowing for an interpretation of these phases.

Integrating mirrors with magnetic components is crucial for constructing chiral optical cavities, which provide tunable platforms for time-reversal-asymmetric light-matter interactions. Here, we introduce single-crystal circular-polarization-selective mirrors based on chiral superconductors, which break time-reversal symmetry themselves, eliminating the need for additional components. We show that a circular-polarization-selective perfect reflection (CSPR) occurs for strong-coupling superconductors in the BCS-BEC crossover regime or beyond if the optical Hall conductivity is significant in the unit of conductivity quantum per unit layer, e2/haz, where az is the lattice constant along the surface normal. While the optical Hall conductivity in chiral superconductors is typically tiny, we classify three routes to obtain a large value. We demonstrate the significant optical Hall conductivity and the resulting CSPR with two examples: (1) superconductivity in doped quantum Hall insulators and (2) chiral pairing that preserves the Bogoliubov Fermi surfaces in the weak-pairing limit. We also discuss the application of our theory to the recently discovered chiral superconducting phase in rhombohedral graphene. Our theory reveals the potential of these classes of chiral superconductors as promising elements for building high-quality-factor terahertz chiral cavities.

We propose a theory for the microscopic origin of the multiple superconducting and magnetic phases observed in CeRh_{2}As_{2} based on the existence of Van Hove singularities near the Fermi energy. The nonsymmorphic symmetry of this material implies that these singularities are located away from high-symmetry momenta; i.e., they have so-called type-II character. This allows us to include the significant Rashba spin-orbit coupling in CeRh_{2}As_{2} in a parquet renormalization group approach. When Fermi-surface nesting is strong, our analysis reveals two closely competing superconducting states with opposite parities, as well as an instability toward spin-density wave states that support both of them, consistent with the phase diagram of CeRh_{2}As_{2}. Type-II Van Hove singularities are generic to nonsymmorphic space groups, and so our theory implies that many other compounds may support closely competing even- and odd-parity superconductivity.

This work systematically examines the influence of partial substitution of calcium (Ca) by erbium (Er) on the structural integrity and superconducting performance of Bi2Sr2Ca2Cu3O10+δ (BSCCO) high-temperature superconductors. Samples were synthesized through the conventional solid-state reaction method and characterized via X-ray diffraction, scanning electron microscopy, resistivity, and magnetization measurements. Despite the close effective ionic radii of Er and Ca, experimental results revealed that increasing Er content led to a deterioration of the superconducting phase and the emergence of semiconducting characteristics. Quantitative analyses demonstrated a notable reduction in critical current density ( Jc), with undoped BSCCO samples exhibiting approximately 14% higher Jc values under equivalent conditions. These findings suggest that while ionic size considerations may permit Er substitution from a stoichiometric perspective, the structural and superconducting phase stability of the BSCCO matrix is adversely affected by Er incorporation.

Universal quasidegenerate orbital origin of two-dome phases in iron pnictide superconductors

The interplay of superconducting and topological states in two-dimensional (2D) materials has gained intensive attention for exploring novel quantum phenomena and their applications in quantum computing. However, 2D materials exhibiting both superconductivity and topological phases are exceptionally rare. In this context, we investigated 2D CrH2(chromium dihydride) in P-6m2 (hexagonal) and P-3m1 (trigonal) symmetries using first-principles calculations. We verified the stability of these phases using phonon dispersion and mechanical stability analyses. Based on ourZ2invariant calculations, CrH2is topologically nontrivial for the P-6m2 symmetry, while it is topologically trivial for the P-3m1 symmetry. Using anisotropic Migdal-Eliashberg equations, we find both phases as single-gap superconductors, with transition temperatures of ∼11 K for the hexagonal phase and ∼8 K for the trigonal phase. The superconducting properties are attributed to electron-phonon coupling between Cr-dorbitals and low-energy phonon modes dominated by Cr vibrations. Our findings offer a promising foundation for further exploration of co-existence of topological and superconducting states in monolayer hydrides and their experimental realization.

We study superconductivity in Ammann-Beenker quasicrystals under magnetic field. By assuming an intrinsic s-wave pairing interaction and solving for mean-field equations self-consistently, we find gapless superconductivity in the quasicrystals at and near half filling. We show that gapless superconductivity results from the combination of broken translational symmetry and confined states that is characteristic of the quasicrystals. When Rashba spin-orbit coupling is present, the quasicrystalline gapless superconductor can be topologically nontrivial and characterized by a nonzero pseudospectrum invariant given by a spectral localizer. The gapless topological superconducting phase exhibits edge states with near-zero energy. These findings suggest that quasicrystals can be a unique platform for realizing gapless superconductivity with nontrivial topology. Published by the American Physical Society 2025

Abstract Strongly correlated systems famously show intriguing (and unexpected) phenomena. The layered 1T‐TaS₂ is no exception, showing different charge density wave configurations, metal insulator transitions (MITs), a fascinating superconducting phase, and low‐temperature meta‐stable hidden phases upon light or current pulses. And now – also a non‐volatile memory effect. The memory forms following cooling of the sample to a chosen temperature in the metal‐insulator coexisting phase of the Mott MIT (≈180 K) then ramping back up to the metallic state. It manifests as a resistance decrease in the following R versus T measurement that is largest at the ramp‐reversal temperature. The memory disappears after cooling to lower temperatures and the original R versus T is recovered. Memory properties are shown to coincide with those of the ramp reversal memory (RRM) previously reported in correlated oxides, including non‐volatility and the ability to write more than one memory. However, there are notable differences and a different origin. These findings indicate that RRM extends beyond oxides, highlighting its universality in correlated materials having the necessary ingredients: phase transition with spatial phase coexistence and a mechanism that locally modifies the transition properties. These findings open new opportunities for exploring memory phenomena in correlated materials.

Abstract Two-dimensional (2D) and three-dimensional (3D) polycrystalline vanadium films grown on SiO2 substrates have been prepared by magnetron sputtering method. The superconductor-insulator transition (SIT) has been observed by adjusting the film thickness, average crystallite size and magnetic field respectively. The complete superconducting 3D vanadium film deposited at 523 K has an obvious two-step development of superconductivity when a magnetic field is applied. This behavior is attributed to non-uniform coupling due to variations in grain sizes and the intergranular distances across the sample. The metallic state induced by the magnetic field can be explained by a model that incorporates both thermally activated phase slippage and quantum phase slippage. Both the 2D vanadium film with thickness of 12 nm and the 3D vanadium film with average crystallite size of 11.9 nm exhibit a trend towards superconducting transition where the resistance decreases to a nonzero value. The SIT, tuned by adjusting the perpendicular magnetic field, has been observed in the above two films. The scaling behavior near the critical magnetic field B c distinguishes a superconducting phase from an insulating phase, indicating a quantum phase transition induced by a finite-size effect. The upper critical magnetic field H c 2 (0) increases as the grain size decreases, likely due to a reduction in the coherence length resulting from a decrease in the average free path with decreasing size.