Mechanism Of Superconductivity Research Articles

Unlikelihood of a phonon mechanism for the high-temperature superconductivity in La3Ni2O7

The discovery of ~80 K superconductivity in nickelate La3Ni2O7 under pressure has ignited intense interest. Here, we present a comprehensive first-principles study of the electron-phonon (e-ph) coupling in La3Ni2O7 and its implications on the observed superconductivity. Our results conclude that the e-ph coupling is too weak (with a coupling constant λ ≲ 0.5) to account for the high Tc, albeit interesting many-electron correlation effects exist. While Coulomb interactions (via GW self-energy and Hubbard U) enhance the e-ph coupling strength, electron doping (oxygen vacancies) introduces no major changes. Additionally, different structural phases display varying characteristics near the Fermi level, but do not alter the conclusion. The e-ph coupling landscape of La3Ni2O7 is intrinsically different from that of infinite-layer nickelates. These findings suggest that a phonon-mediated mechanism is unlikely to be responsible for the observed superconductivity in La3Ni2O7, pointing instead to an unconventional nature.

Superconductivity in an infinite-layer nickelate superlattice

Recent observations of superconductivity in infinite-layer nickelates offer insights into high-temperature superconductivity mechanisms. However, defects and dislocations in doped films complicate the realization of superconductivity, limiting current research to superconducting nickelate heterostructures. The lack of research on superconductivity in superlattices composed of ultrathin nickelates severely impedes not only the exploration of the interface effect on superconductivity, but also the utilization of heterostructure engineering for exploring higher superconducting temperature Tc. Here, we demonstrated superconducting infinite-layer nickelate superlattices [(Nd0.8Sr0.2NiO2)8/(SrTiO3)2]10 via topotactic reduction. Our study uncovered that only above a critical thickness can high-quality superlattices be achieved, with structural formation dependent on nickelate layer thickness. The superconducting superlattice showed a Tc of 12.5 K and a 2D superconducting feature, indirectly indicate the intrinsic superconductivity of infinite-layer nickelates. Our study offers promising avenues for delving into the superconducting mechanism and for exploring multilevel interface engineering of infinite-layer nickelates, thus opening new horizons for the study of infinite-layer nickelates.

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

The Superconducting Mechanism in BiS2-Based Superconductors: A Comprehensive Review with Focus on Point-Contact Spectroscopy.

The family of BiS2-based superconductors has attracted considerable attention since their discovery in 2012 due to the unique structural and electronic properties of these materials. Several experimental and theoretical studies have been performed to explore the basic properties and the underlying mechanism for superconductivity. In this review, we discuss the current understanding of pairing symmetry in BiS2-based superconductors and particularly the role of point-contact spectroscopy in unravelling the mechanism underlying the superconducting state. We also review experimental results obtained with different techniques including angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, specific heat measurements, and nuclear magnetic resonance spectroscopy. The integration of experimental results and theoretical predictions sheds light on the complex interplay between electronic correlations, spin fluctuations, and Fermi surface topology in determining the coupling mechanism. Finally, we highlight recent advances and future directions in the field of BiS2-based superconductors, underlining the potential technological applications.

High‐Pressure Growth and Characterization of Seven‐Layered CuBa2Ca6Cu7O17±δ Single Crystals

AbstractIn cuprates, the superconducting transition temperature (Tc) is closely related to the number n of CuO2 planes per unit cell. The studies on multi‐layered cuprates offer insights into the physical properties and pairing mechanisms of superconductivity. However, the synthesis and stability of ultra‐multilayered cuprates pose significant challenges. Here, a newly discovered seven‐layered cuprate CuBa2Ca6Cu7O17±δ grown under high pressure is reported. Magnetization and transport measurements confirm bulk superconductivity with Tc of ≈85 K. The magnetization hysteresis loops, critical current density (Jc), and irreversibility fields of CuBa2Ca6Cu7O17±δ are also investigated. The novel compound CuBa2Ca6Cu7O17±δ provides an ideal platform for studying the physics of multi‐layered cuprates.

Atomic-Scale Mechanism of Enhanced Electron-Phonon Coupling at the Interface of MgB2 Thin Films.

In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, electron-phonon coupling is the fundamental mechanism of superconductivity. For instance, the superconductivity of magnesium diboride (MgB2) comes from the coupling between E2g modes (in-plane boron-boron bond vibrations) and self-doped charge carriers. In thin films and ceramics of BCS superconductors, interfaces with discontinuous chemical bonds may alter the local electron-phonon coupling. However, such effects remain largely unexplored. Here, we investigate the heterointerface of the MgB2 film on the SiC substrate at the atomic scale using electron microscopy and spectroscopy. We detect the presence of a thin MgO layer with a thickness of ∼1 nm between MgB2 and SiC. Atomic-level electron energy loss spectra (EELS) show MgB2-E2g mode splitting and softening near the MgB2/MgO interface, which enhances electron-phonon coupling at the interface. Our findings highlight the potential of interface engineering to enhance superconductivity via modulating local phonon states and/or electron states.

THz optical response of Ba(Fe[formula omitted]Ni[formula omitted])[formula omitted]As[formula omitted] films analyzed within the three-band Eliashberg s[formula omitted]-wave model

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe1−xNix)2As2 thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe1−xNix)2As2 is a multiband superconductor with s±-wave pairing symmetry.

Anomalous Frequency and Temperature Dependent Scattering in the Dilute Metallic Phase in Lightly Doped SrTiO_{3}.

The mechanism of superconductivity in materials with aborted ferroelectricity and its emergence out of a dilute metallic phase in systems like doped SrTiO_{3} is an outstanding issue in condensed matter physics. This dilute metal has anomalous properties that are both similar and different to those found in the normal state of other unconventional superconductors. For instance, T^{2} resistivity can be found at densities that are too small to allow current decay through electron-electron scattering. We have investigated the optical properties of the dilute metallic phase in doped SrTiO_{3} using THz time-domain spectroscopy. At low frequencies the THz response exhibits a Drude-like form as expected for typical metal-like conductivity. We observed the frequency and temperature dependencies to the low energy scattering rate Γ(ω,T)∝(ℏω)^{2}+(pπk_{B}T)^{2} expected in a conventional Fermi liquid. However, we find the lowest known p values of 0.39-0.72. As p is 2 in a canonical Fermi liquid and existing models based on energy dependent elastic scattering bound p from below to 1, our observation lies outside current explanation. Our data also give insight into the high temperature regime and show that the temperature dependence of the resistivity derives in part from strong T dependent mass renormalizations.

Realization of hydrogenation-induced superconductivity in two-dimensional Ti2N MXene.

Two-dimensional (2D) MXene superconductors have been currently attracting considerable interest due to their unique electronic properties and diverse applicability. Utilizing first-principles computational methods, we have designed two distinct configurations of hydrogenated 2D Ti2N MXene materials, namely Ti2NH2 and Ti2NH4, and have conducted an exhaustive analysis of their structural stability, electronic characteristics, and superconductivity. Hydrogenation endows monolayer Ti2N with inherent metallic characteristics, as evidenced by an elevated density of states (DOS) at the Fermi level (Ef). Notably, Ti2NH4 exhibits a superconducting critical temperature (Tc) of 15.8 K, which is predominantly ascribed to the electronic contributions stemming from the Ti 3d orbitals. Analysis of phonon dispersion underscores the pivotal role that diverse lattice vibrational modes play in electron-phonon coupling (EPC), particularly the significance of low-frequency vibrations for facilitating electron pairing and the emergence of superconductivity. Furthermore, strain engineering can effectively modulate the superconducting properties of Ti2NH4, with a 2% tensile strain enhancing the EPC strength (λ) to 0.857 and increasing Tc to 18.7 K. This research elucidates the superconducting mechanisms of hydrogenated Ti2N structures, offering valuable insights for the development of novel 2D superconducting materials.

Optimisation of pulsed laser deposited Ba1-xKxBiO3 thin films with tunable superconducting properties by control of K doping level, x

The mid-TC superconductor Ba1-XKXBiO3 (BKBO) exhibits different superconducting mechanisms depending on x, in the range ∼ 0.35–0.65. The optimal doping for the highest TC is reported to be around x = 0.4. To understand more about the dependence of the superconducting mechanism on x, high quality and reproducible epitaxial films with controlled x are needed. This has been challenging owing to the volatility of K and (to a lesser extent) Bi. In this work, we use pulsed laser deposition (PLD) with several novel process steps to achieve high-quality films in a reproducible way. These include a modified method for target preparation, a low NO2 growth pressure, and precise positioning of substrates in the PLD plume. Optimum TC films (32 K onset) were grown from an x = 0.4 target, i.e. with no excess K, as is normally used. Stable, higher K content films (made from an x = 0.45 target), were also grown. These x = 0.45 films had a lower TC (22.5 K onset), as expected for (K) overdoped films, with very high upper critical field, HC2 (0 K), and irreversibility field, Hirr (0 K), values, from linear extrapolation, of ∼ 31.7 T and ∼ 28.8 T, respectively. The growth methodology demonstrated in this work is highly beneficial for fundamental mechanistic studies of this complex superconductor on which there is renewed interest, and where controlled compositions and crystalline quality are currently limited.

Isotropic Quantum Griffiths Singularity in Nd_{0.8}Sr_{0.2}NiO_{2} Infinite-Layer Superconducting Thin Films.

This work reports on the emergence of quantum Griffiths singularity (QGS) associated with the magnetic field induced superconductor-metal transition (SMT) in unconventional Nd_{0.8}Sr_{0.2}NiO_{2} infinite layer superconducting thin films. The system manifests isotropic SMT features under both in-plane and perpendicular magnetic fields. Importantly, after scaling analysis of the isothermal magnetoresistance curves, the obtained effective dynamic critical exponents demonstrate divergent behavior when approaching the zero-temperature critical point B_{c}^{*}, identifying the QGS characteristics. Moreover, the quantum fluctuation associated with the QGS can quantitatively explain the upturn of the upper critical field around zero temperature for both the in-plane and perpendicular magnetic fields in the phase boundary of SMT. These properties indicate that the QGS in the Nd_{0.8}Sr_{0.2}NiO_{2} superconducting thin film is isotropic. Moreover, a higher magnetic field gives rise to a metallic state with the resistance-temperature relation R(T) exhibiting lnT dependence among the 2-10K range and T^{2} dependence of resistance below 1.5K, which is significant evidence of Kondo scattering. The interplay between isotropic QGS and Kondo scattering in the unconventional Nd_{0.8}Sr_{0.2}NiO_{2} superconductor can illustrate the important role of rare region in QGS and help to uncover the exotic superconductivity mechanism in this system.

Antiferromagnetic phase transition in a 3D fermionic Hubbard model.

The fermionic Hubbard model (FHM)1 describes a wide range of physical phenomena resulting from strong electron-electron correlations, including conjectured mechanisms for unconventional superconductivity. Resolving its low-temperature physics is, however, challenging theoretically or numerically. Ultracold fermions in optical lattices2,3 provide a clean and well-controlled platform offering a path to simulate the FHM. Doping the antiferromagnetic ground state of a FHM simulator at half-filling is expected to yield various exotic phases, including stripe order4, pseudogap5, and d-wave superfluid6, offering valuable insights into high-temperature superconductivity7-9. Although the observation of antiferromagnetic correlations over short10 and extended distances11 has been obtained, the antiferromagnetic phase has yet to be realized as it requires sufficiently low temperatures in a large and uniform quantum simulator. Here we report the observation of the antiferromagnetic phase transition in a three-dimensional fermionic Hubbard system comprising lithium-6 atoms in a uniform optical lattice with approximately 800,000 sites. When the interaction strength, temperature and doping concentration are finely tuned to approach their respective critical values, a sharp increase in the spin structure factor is observed. These observations can be well described by a power-law divergence, with a critical exponent of 1.396 from the Heisenberg universality class12. At half-filling and with optimal interaction strength, the measured spin structure factor reaches 123(8), signifying the establishment of an antiferromagnetic phase. Our results provide opportunities for exploring the low-temperature phase diagram of the FHM.

Interfacial Electron Beam Lithography Converts an Insulating Organic Monolayer to a Patterned Single-Layer Conductor with Puzzling Charge Transport Performance.

The direct generation of conducting paths within an insulating surface represents a conceptually unexplored approach to single-layer electrical conduction that opens vistas for exciting research and applications fundamentally different from those based on specific layered materials. Herein we report surface channels with single-layer -COOH functionality patterned on insulating n-octadecyltrichlorosilane monolayers on silicon that exhibit unusual ionic-electronic conduction when equipped with ion-releasing silver electrodes. The strong dependence of charge transport in such channels on their lateral dimensions (nanosize, macro-size), the type (p, n) and resistivity (doping level) of the underlying silicon substrate, the nature of the insulating spacer layer between the conducting channel and the silicon surface, and the postpatterning chemical manipulation of channel's -COOH functionality allows designing channels with variable resistivities, ranging from that of a practical insulator to some unexpectedly low values. The unusually low resistivities displayed by channels with nanometric widths and micrometer-millimeter lengths are attributed primarily to enhanced electronic transport within ultrathin nanowire-like silver metal films formed along their conductive paths. Function-structure correlations derived from a comprehensive analysis of electrical, atomic force microscopy, and Fourier transform infrared spectral data suggest an unconventional mode of conduction in these channels, which has yet to be elucidated, apparently involving coupled ionic-electronic transport mediated and enhanced by interfacial electrical interactions with charge carriers located outside the conducting channel and separated from those carrying the measured current. These intriguing findings hint at effects akin to Coulomb pairing in the proposed mechanisms of excitonic superconductivity in interfacial nanosystems structurally related to the present metalized surface channels.

Electron, phonon, and superconducting properties of Mg2CuH6 under pressure

Recent theoretical prediction of Mg2IrH6 is a significant advance in achieving high-temperature superconductivity under atmospheric pressure, Mg2IrH6 is the hydride superconductor with the highest superconducting transition temperature (Tc ∼ 160 K) under ambient pressure so far. In general, Tc is usually related to the degree of hydrogen enrichment. As a representative of the hydrogen-poor structures, MgHCu3 was also predicted to have a Tc of 43 K under atmospheric pressure. In this work, we try to replace the Ir atom in Mg2IrH6 with the Cu atom to improve the superconductivity of the system. The phonon dispersion curves and the ab initio molecular dynamics (AIMD) simulation indicate that the novel ternary Mg2CuH6 hydride is dynamically and thermodynamically stable. But, regrettably, the Tc and electron-phonon coupling (EPC) parameters λ of Mg2CuH6 are calculated to be 12.5 K and 0.55 at 25 GPa, which is far inferior to the superconductivity of Mg2IrH6. Compared with Fm3¯m Mg2CuH6, Fm3¯m Mg2IrH6 produces obvious phonon softening in the G point near 30 meV, which significantly enhances the EPC and increases the Tc of the system. Moreover, Cu substitution for Ir results in a sharp decrease in the contribution of the electronic states for Mg and H to the total density of states at the Fermi level. The huge difference in predicted Tc between Mg2CuH6 and Mg2IrH6 and their causes may provide insights into the design of high-temperature atmospheric superconductors and help to understand the superconducting mechanisms of related systems.

Strain-induced long-range charge-density wave order in the optimally doped Bi2Sr2−xLaxCuO6 superconductor

The mechanism of high-temperature superconductivity in copper oxides (cuprate) remains elusive, with the pseudogap phase considered a potential factor. Recent attention has focused on a long-range symmetry-broken charge-density wave (CDW) order in the underdoped regime, induced by strong magnetic fields. Here by 63,65Cu-nuclear magnetic resonance, we report the discovery of a long-range CDW order in the optimally doped Bi2Sr2−xLaxCuO6 superconductor, induced by in-plane strain exceeding ∣ε∣ = 0.15 %, which deliberately breaks the crystal symmetry of the CuO2 plane. We find that compressive/tensile strains reduce superconductivity but enhance CDW, leaving superconductivity to coexist with CDW. The findings show that a long-range CDW order is an underlying hidden order in the pseudogap state, not limited to the underdoped regime, becoming apparent under strain. Our result sheds light on the intertwining of various orders in the cuprates.

Interplay of superconductivity and magnetism in the Fe1+yTe1‐xSex iron‐based superconductor: A theoretical study

AbstractThe competition between superconductivity and magnetism has been studied for the ongoing investigation of mechanisms in unconventional superconductors. Fe1+yTe1−xSex is distinct among iron‐based superconductors due to its structural simplicity, comprising solely FeTe/Se layers that are favorable for probing the superconductivity mechanism. The main goal of this article is to focus on the computational investigation of the interplay of magnetism and superconductivity in the Fe1+yTe1−xSex multiband superconductor. From the electronic structure of the Fe1+yTe1−xSex superconductor, a two‐band Hamiltonian model was considered to examine the interplay of magnetism and superconductivity in the Fe1+yTe1‐xSex superconductor. Using Green's function formalism with the Hamiltonian model, the mathematical statements for the superconducting (Sc) gap parameter, spin density wave (SDW) ordering parameter, Sc and magnetic transitional temperatures have been obtained. Using these mathematical expressions, the phase diagram of transition temperatures versus the gap parameters for the Fe1+yTe1‐xSex superconductor has been plotted. The intersection region in the phase diagram of transition temperatures as a function of the SDW order parameter has been plotted to show the possible interplay of superconductivity and magnetism in a Fe1+yTe1‐xSex iron‐based superconductor.

Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

Nontrivial d-electrons driven superconductivity of transition metal diborides

Leveraging the progress of first-principles modelings in understanding the mechanisms of superconductivity of materials, in this work we investigate the phonon-mediated superconducting properties of transition metal diborides. We report that TaB2 and NbB2 show superconducting transition temperatures as high as 27.0 and 26.0 K at ambient conditions, respectively, comparable with those obtained for CaB2 or MgB2. By mode-by-mode analysis of the electron-phonon-coupling, we reveal that the high superconducting temperature of transition metal diborides is due mainly to the strong coupling between d electrons of the transition metals and the acoustic phonon modes along out-of-plane vibrations. This fact is distinct from that of CaB2 or MgB2, where the superconductivity stems mainly from the boron px and py orbitals, which couple strongly to the optical phonon modes dominated by in-plane B atomic vibrations. Further, we find that transition metal diborides present only a superconducting gap at low temperatures, whereas CaB2 or MgB2 are double superconducting gap superconductors. In addition, we investigate the strain effect on the superconducting transition temperatures of diborides, predicting that Tc can be further enhanced by optimizing the phonon and electronic interactions. This study sheds some light on the exploring high Tc boron-based superconductor materials.

Superconductivity in Dilute Hydrides of Ammonia under Pressure.

The past decade has witnessed great progress in predicting and synthesizing polyhydrides that exhibit superconductivity under pressure. Dopants allow these compounds to become metals at pressures lower than those required to metallize elemental hydrogen. Here, we show that by combining the fundamental planetary building blocks of molecular hydrogen and ammonia, conventional superconducting compounds can be formed at high pressure. Through extensive theoretical calculations, we predict metallic metastable structures with NHn (n = 10, 11, 24) stoichiometries that are based on NH4+ superalkali cations and complex hydrogenic lattices. The hydrogen atoms in the molecular cation contribute to the superconducting mechanism, and the estimated superconducting critical temperatures, Tc's, are comparable to the highest values computed for the alkali metal polyhydrides. The largest calculated (isotropic Eliashberg) Tc is ∼180 K for Pnma-NH10 at 300 GPa. Our results suggest that other molecular cations can be mixed with hydrogen under pressure, yielding superconducting compounds.

ARPES detection of superconducting gap sign in unconventional superconductors

The superconducting gap symmetry is crucial in understanding the underlying superconductivity mechanism. Angle-resolved photoemission spectroscopy (ARPES) has played a key role in determining the gap symmetry in unconventional superconductors. However, it has been considered so far that ARPES can only measure the magnitude of the superconducting gap but not its phase; the phase has to be detected by other phase-sensitive techniques. Here we propose a method to directly detect the superconducting gap sign by ARPES. This method is successfully validated in a cuprate superconductor Bi2Sr2CaCu2O8+δ with a well-known d-wave gap symmetry. When two bands have a strong interband interaction, the resulted electronic structures in the superconducting state are sensitive to the relative gap sign between the two bands. Our present work provides an approach to detect the gap sign and can be applied to various superconductors, particularly those with multiple orbitals like the iron-based superconductors.