Mechanism Of high-Tc Superconductivity Research Articles

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.

Planckian behavior of cuprates at the pseudogap critical point simulated via flat electron-boson spectral density

Planckian behavior has been recently observed in La1·76Sr0·24CuO4 at the pseudogap critical point. The Planckian behavior takes place in an intriguing quantum metallic state at a quantum critical point. Here, the Planckian behavior was simulated with an energy-independent (or flat) and weakly temperature-dependent electron-boson spectral density (EBSD) function by using a generalized Allen's (Shulga's) formula. We obtained various optical quantities from the flat EBSD function, such as the optical scattering rate, the optical effective mass, and the optical conductivity. These quantities are well fitted with the recently observed Planckian behavior. Fermi-liquid behavior was also simulated with an energy-linear and temperature-independent EBSD function. The EBSD functions agree well with the overall doping- and temperature-dependent trends of the EBSD function obtained from the optically measured spectra of cuprate systems, which can be crucial for understanding the microscopic electron-pairing mechanism for high-Tc superconductivity in cuprates.

Signatures of superconductivity near 80 K in a nickelate under high pressure.

Although high-transition-temperature (high-Tc) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown1-4. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with Tc above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La3Ni2O7 with maximum Tc of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the [Formula: see text] and [Formula: see text] orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the [Formula: see text] orbitals with the apical oxygen ions connecting the Ni-O bilayers. Thus, our discoveries provide not only important clues for the high-Tc superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-Tc superconductivity mechanism.

Possible Manifestation of Q-Ball Mechanism of High-Tc Superconductivity in X-ray Diffraction

It is demonstrated, that recently proposed by the author Q-ball mechanism of the pseudogap state and high-Tc superconductivity in cuprates may be detected in micro X-ray diffraction, since it imposes inverse correlations between the size and scattering intensities of the Q-ball charge-density-wave (CDW) fluctuations in these compounds. The Q-ball charge Q gives the number of condensed elementary bosonic excitations in a CDW fluctuation of finite amplitude. The attraction between these excitations inside Euclidean Q-balls is self-consistently triggered by the simultaneous condensation of Cooper/local pairs. Euclidean Q-ball solutions, analogous to the famous Q-balls of squarks in the supersymmetric standard model, arise due to the global invariance of the effective theory under the U(1) phase rotation of the Fourier amplitudes of the short-range CDW fluctuations. A conserved ‘Noether charge’ Q along the Matsubara time axis equals Q∝TM2V, where the temperature T, Q-ball’s volume V, and fluctuation amplitude M enter. Several predictions are derived in an analytic form that follow from this picture. The conservation of the charge Q leads to an inverse proportionality between the volume V and X-ray scattering intensity ∼M2 of the CDW puddles found in micro X-ray scattering experiments. The theoretical temperature dependences of the most probable Q value of superconducting Q-balls and their size and scattering amplitudes fit well the recent X-ray diffraction data in the pseudogap phase of high-Tc cuprates.

On the uniaxial strain (pressure) derivatives of the critical temperature of superconductivity of La[formula omitted]Sr[formula omitted]CuO4

The uniaxial strain (and pressure) derivatives of the critical temperature of superconductivity (Tc) of La2−xSrxCuO4 (LSCO) cuprate is studied within the extended Holstein–Hubbard model and bipolaronic mechanism of high-Tc superconductivity. The Tc of LSCO cuprate is associated with the Bose–Einstein condensation temperature, TBEC, of an ideal gas of the intersite bipolarons. Working on a modified chain model lattice of cuprates we expressed TBEC as a function of LSCO’s parameters (apical oxygen ion’s mass and its vibration frequency, the periods and the strains of the lattice). Using the experimental values of LSCO parameters the uniaxial strain derivatives and the uniaxial pressure derivatives of Tc i.e. ∂TBEC/∂ɛi and ∂TBEC/∂pi (i=a,c) of LSCO cuprate are calculated as a function of doping level x. The obtained results are compared with the available experimental data. Qualitative and quantitative similarities as well as differences in the doping dependencies of between our ∂TBEC/∂ɛi (∂TBEC/∂pi) and the experimental ∂Tc/∂ɛi (∂Tc/∂pi) (i=a,c) are discussed. Upon carried analysis we concluded that the uniaxial strain (pressure) derivatives of Tc of LSCO cuprate can be interpreted within the bipolaronic mechanism of superconductivity.

Excess Carrier Density and the Role of Spin Density Waves in the Mechanism of High Tc Superconductivity in (Cu0.5Tl0.5)Ba2Ca2Cu3-xKxO10-δ (x = 0, 1, 2, 2.5, 3) Samples

Excess Carrier Density and the Role of Spin Density Waves in the Mechanism of High Tc Superconductivity in (Cu0.5Tl0.5)Ba2Ca2Cu3-xKxO10-δ (x = 0, 1, 2, 2.5, 3) Samples

Infinite-Layer Nickelate Superconductors: A Current Experimental Perspective of the Crystal and Electronic Structures

After the reward of more than 2 decades of pursuit on the high-Tc cuprate analog with the hope to obtain a better understanding of the mechanism of high-Tc superconductivity, the discovery of superconductivity in the infinite-layer nickelate brings more mystery to the picture than expected. Tops in the list of questions are perhaps 1) absence of superconductivity in the bulk nickelate and limited thickness of the infinite-layer phase in thin film, 2) absence of superconductivity in the La-nickelate despite it being the earliest studied rare-earth nickelate, and the role of 4 f orbital in the recipe of superconductivity, 3) absence of Meissner effect and suspect of the origin of superconductivity from the interface, 4) whether nickelate hosts similar pairing symmetry to the single-band high-Tc cuprates or multiband iron-based superconductor. In this perspective article, we will discuss the following aspects: 1) stabilization of the infinite-layer phase on the SrTiO3(001) substrate and the thickness dependency of observables; 2) rare-earth dependence of the superconducting dome and phase diagram of the (La/Pr/Nd)- infinite-layer nickelate thin film; 3) experimental aspects of the measurement of Meissner effect; 4) theoretical framework and experimental study of the pairing symmetry of infinite-layer nickelate superconductor.

Strongly correlated superconductor with polytypic 3D Dirac points

Topological superconductors should be able to provide essential ingredients for quantum computing, but are very challenging to realize. Spin–orbit interaction in iron-based superconductors opens the energy gap between the p-states of pnictogen and d-states of iron very close to the Fermi level, and such p-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe1−xSex making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1−xSex (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe1−xSex can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the p–d and d–d interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-Tc superconductivity in IBS.

Enhanced Anharmonic Oscillations in Cu0.5Tl0.5Ba2(Ca2−yMgy)Cu3−xCdxO10−δ (y = 0, 1; x = 0, 1.5) Superconductors

Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (undoped), Cu0.5Tl0.5Ba2(CaMg)Cu3O10−δ (Mg-doped), Cu0.5Tl0.5Ba2Ca2Cu1.5Cd1.5O10−δ (Cd-doped), and Cu0.5Tl0.5Ba2(CaMg)Cu1.5Cd1.5O10−δ (Cd/Mg-codoped) samples were synthesized by two-step solid-state reaction at 860°C to investigate the possible role of anharmonic oscillations of atoms in the mechanism of high critical temperature (Tc) superconductivity. The samples showed orthorhombic crystal structure with smaller and larger unit cell volume for the Mg- and Cd/Mg-codoped samples, respectively, compared with the undoped sample. Tc observed by resistivity measurements decreased in the Cd-doped samples, but the maximum suppression was observed in the Cd/Mg-codoped sample showing semiconductor behavior. Fourier-transform infrared (FTIR) absorption spectroscopy revealed hardening of apical oxygen modes on the incorporation of Mg atoms as compared with the undoped sample and similarly with the doping of Mg atoms in the Cd-doped sample, confirming the intrinsic inclusion of these elements in the final compound. Excess conductivity analysis revealed suppression in the coherence length (ξc(0)), interlayer coupling (J), phase relaxation time (τφ) of carriers, Fermi velocity (VF) of carriers, Ginzburg–Landau (GL) parameter κ, breaking energy (Ebreak), and magnetic field penetration depth (λp.d) in Cd-doped samples. The values of Bc0(T), Bc1(T), and Jc(0) increased in the Cd-doped samples, showing enhancement of flux-pinning characteristics. It is proposed that Cd doping at CuO2-planar site induces anharmonic oscillation in the solid-state medium, which suppresses the density of phonons and thereby the density of Cooper pairs and hence superconductivity. These studies emphasize the essential role of electron–phonon interactions in the mechanism of high-Tc superconductivity.

An-harmonic Oscillations and Superconductivity in (Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) Samples

(Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) samples are synthesized for the investigation of role of an-harmonic oscillations in the mechanism of high Tc superconductivity. In the conducting planes, the carrier density is enhanced by K doping at the charge reservoir layer (Cu0.5Tl0.5)Ba2O4-δ by synthesizing (Cu0.25K0.25Tl0.5)Ba2Ca2Cu3-y-zZnyCdzO10-δ (y = z = 0, 0.25, 0.5, 0.75, 1) samples. The (Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) samples were prepared by employing two-step solid-state reaction method accomplished at 860 °C and understand their superconducting properties by resistivity measurements, X-ray diffraction analysis, and FTIR absorption measurements. The orthorhombic unit cell of our samples was confirmed via XRD analysis. The analysis also reflected the increase in c-axis length and the volume of the unit cell with the enhanced dopants (Cd and Zn) concentration. The suppression of Tc(onset) and Tc(R = 0) was found with the increasing concentration of Zn and Cd. An improvement in the Tc(onset) observed in samples doped by K that arises due to increase in the density of free carriers supplied by alkali atoms. With the increasing content of Cd and Zn, the unit cell exhibits the softening of Cu(1)-OA-Cu(2) and Tl-OA-Cu(2) apical oxygen mode and hardening of planar oxygen mode. From FIC analysis, the Tcmf and T* and various cross over temperature were shifted towards lower temperature with the enhanced Zn and Cd dopant concentration. The value of α determining the defect density also increases with the increasing dopant (Zn and Cd) concentration. The numerical values of different parameter like Bc0(T), Bc1(T), Jc(0), and Ebreak showing increasing trend but the phase relaxation time of the carriers τφ points out decreasing trend with the enhanced doping of Cd and Zn. Moreover, these parameters reflected inferior values in K-doped samples relative to K-undoped samples. The value of other parameter like coherence length ξc(0) along c-axis, interlayer coupling J, magnetic field penetration depth λp.d, the Ginzburg Landau (GL) parameter κ, and the Fermi velocity VF of the carriers decreases as doping of Zn and Cd enhanced. These parameters show superior values in K-doped samples relative to K-undoped samples due to increase in the density of free carriers supplied by K atoms. The negative values of real part of dielectric constant suppress in both K undoped and K doped samples with increasing concentration of Cd and Zn. The imaginary part of dielectric constant and ac conductivity showing suppression in their negative values in K undoped samples while showing increase in absolute values in K doped samples with increasing concentration of Cd and Zn.

Effect of Cd intercalation on the superconducting properties of (Cu0.5-yKyTl0.5)Ba2Ca2Cu3-xCdxO10-δ (y = 0, 0.25; x = 0, 0.5, 1.0, 1.5, 2.0) superconductors

(Cu0.5Tl0.5)Ba2Ca2 Cu3-xCdxO10-δ (x = 0, 0.5, 1.0, 1.5, 2.0) samples are synthesized for the investigation of role of an-harmonic oscillations in the mechanism of high Tc superconductivity. The density of carriers in the conducting planes is enhanced with the doping of K at the (Cu0.5Tl0.5)Ba2O4-δ by synthesizing (Cu0.25K0.25Tl0.5)Ba2Ca2Cu3-xCdxO10-δ (x = 0, 0.5, 1.0, 1.5, 2.0) samples. These samples were synthesized by two step solid state reaction method accomplished at 860 °C and studied their superconducting properties by resistivity measurements, X-ray diffraction analysis and FTIR absorption measurements. The planar reflections observed in the XRD analysis of the samples are best fitted to the orthorhombic crystal structure. The c-axis length and the volume of the unit cell increases with the enhanced doping of Cd in the final compound. The onset temperature of superconductivity Tc (onset) and the zero-resistivity critical temperature Tc (R = 0) suppresses with the increased doping of Cd in the final compound. An improvement in the Tc (onset) and Tc (R = 0) is observed in K-doped samples that arises due to increase in the density of free carriers supplied by alkali atoms. A softening of apical oxygen mode of type Cu(1)‒OA‒Cu(2)/Cd is observed with the increased doping of Cd at the CuO2 planar sites. The apical oxygen mode of type Tl‒OA‒Cu(2)/Cd is, however, did not change with the doping of Cd in the final compound. The planar oxygen mode (545 ‒ 570 cm−1) is hardened with the enhanced doping of Cd in the final compound. The excess conductivity analyses of conductivity data have shown a shift in the various cross-over temperatures, Tcmf and T* towards lower temperatures with the enhanced doping of Cd in the final compound. The value of α determining the defect density also increases with the increased doping of Cd in the final compound. The coherence length along the c-axis ξc(0), interlayer coupling J, magnetic field penetration depth λp.d and Ginzburg Landau (GL) parameter κ decreased with the increase doping of Cd. The phase relaxation time of the carriers Ꞇϕ, the Fermi velocity VF of the carriers decreased while Bc0(T), Bc1(T) and Jc (0) are enhanced with increase doing of Cd in the final compound.

Enhanced superconducting properties of Ti doped (Cu0.5Tl0.5)Ba2(Ca2−xTix)Cu3O10−δ samples

Ti-doped (Cu0.5Tl0.5)Ba2(Ca2−xTix)Cu3O10−δ (x = 0, 0.25, 0.50, 0.75, 1.0) superconductors have been synthesized by solid state reaction method. The prepared samples are characterized by XRD, electrical resistivity, AC susceptibility and FTIR techniques. XRD analysis showed that the lattice parameters are marginally altered but there is no substantial change with the doping of Ti. All the samples are found with orthorhombic crystal structure following PMMM space group. Tc(R = 0) is enhanced with the doping of Ti except for x = 1.0 sample. The magnitude of diamagnetism in the AC-susceptibility measurements is enhanced up to x = 0.5 and decreased beyond. The FTIR measurements show that the apical oxygen phonon modes are softened. The planar oxygen phonon modes are softened but their intensity is raised with the enhanced Ti contents. This softening of the planar oxygen phonon modes may be arising due to the heavier Ti ions (47.9 amu) at the lighter Ca (40.07 amu) sites. Variation in the lattice parameters in the XRD data and shifting of various oxygen phonon modes in the FTIR data show that Ti is incorporated at Ca sites in the unit cell. Increase in the superconducting properties up to certain doping level (x = 0.75) may be arising due to the improved interplane coupling caused by the smaller sized Ti at Ca sites. While the decrease in Tc and magnitude of diamagnetism beyond x = 0.75 and x = 0.5 respectively, is attributed to the suppressed density of a particular type of phonons required for optimum superconductivity. This suppression in the density of the desired phonons is brought about by the substitution of heavier Ti ions at the lighter Ca sites. This study signifies the role of electron–phonon interaction in mechanism of high Tc superconductivity.

Superconductivity and fast proton transport in nanoconfined water

A real-space molecular-orbital density-wave description of Cooper pairing in conjunction with the dynamic Jahn–Teller mechanism for high-Tc superconductivity predicts that electron-doped water confined to the nanoscale environment of a carbon nanotube or biological macromolecule should superconduct below and exhibit fast proton transport above the transition temperature, Tc ≅ 230 K (−43 °C).

A self-consistent determination of the RVB and SC gaps in the YRZ ansatz

A correct understanding of the origin of the pseudogap in high temperature (high-Tc) cuprate superconductors is considered to be a peripheral breakthrough in the understanding of the microscopic mechanism of the high-Tc superconductivity. Yang–Rice–Zhang (YRZ) ansatz is an important phenomenological theory to describe the phenomenon of pseudogap. However, in the framework of YRZ, the pseudogap (resonant valence bond (RVB) gap) and the superconducting (SC) gap are unable to have a self-consistent determination at different doping concentrations, and this severely limits the application of the YRZ ansatz. Based on the YRZ ansatz, this study develops a technical method to determine the RVB and SC gaps in a self-consistent manner. It is revealed that the self-consistent calculations of the doping dependence of RVB, SC gaps and spectral function are not only consistent with the empirical gap formula in the YRZ framework, but also consistent with the doping evolution of the Fermi surface observed in the angle-resolved photoemission spectroscopy (ARPES) experiments. Our method will greatly extend the applications of the YRZ ansatz, and will deepen our understanding of the origin of pseudogap as well as the mechanism of high-Tc superconductivity.

The Role of Mass of Doped Atoms in the Superconductivity of Cu0.5Tl0.5Ba2Ca2Cu3O10−d and Cu0.5Tl0.5Ba2Ca2Cu1.5 M1.5O10−d (M = Cd, Zn, and Ni)

For the investigation of any possible role of the mass of atoms in the mechanisms of higher T c superconductivity, we have synthesized Cu0.5Tl0.5Ba2Ca2Cu3O10−d and Cu0.5Tl0.5Ba2Ca2Cu1.5 M 1.5O10−d (M = Cd, Zn, and Ni) samples by two-step solid-state reaction method at 860°C. The synthesized samples have shown orthorhombic crystal structure, and the unit cell volume increases with the doping of M atoms. A suppression in the onset temperature of superconductivity and zero-resistivity critical temperature is observed with the doping of Cd; whereas, these parameters are enhanced with the doping of Ni and Zn. In FTIR absorption measurements, the apical oxygen mode of type Tl–OA–Cu(2) mode is hardened with the incorporation of M = Cd, Zn, and Ni atoms, confirming the doping of these elements in the final compound. In excess conductivity analyses, a suppression is observed in the values of coherence length ξ c (0), inter-layer coupling J and the Fermi velocity V F of carriers with the doping of Cd; whereas the values of these parameters are enhanced with Zn and Ni doping. However, Cd atoms have also been found to induce the enhancements of flux-pinning characters as B c0 (T), B c1 (T), and J c (0); whereas the values of these parameters are suppressed in Ni- and Zn-doped samples. A suppression of magnetic field penetration depth (λ p.d) and the Ginzburg-Landau (GL) parameter κ are observed in Cd-doped samples. The energy required to break apart the Cooper pairs is suppressed in the Cd-doped samples showing that Cd-doped samples promotes weak superconductivity These studies confirm that doped Cd atoms induce anharmonic oscillations in the solid-state medium which suppress the density of phonons and hence the density of Cooper pairs that stress the importance of electron-phonon interaction in the mechanism of high T c superconductivity.

Influence of Ti doping on the superconducting properties of YBa2(Cu3−xTix)O7−δ materials

YBa2(Cu3−xTix)O7−δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) superconductors are synthesized at 900 °C by solid state reaction method. The superconducting properties of the samples are studied by X-ray diffraction, resistivity, AC-susceptibility and Fourier transform infrared spectroscopy (FTIR) absorption measurements. These samples have shown orthorhombic crystal structure and the cell parameters slightly increase for higher Ti-contents. The Tc(R = 0), the onset temperature and magnitude of diamagnetism are suppressed with increasing Ti-doping. A new in-chain [Ti–O–Cu(1)] oxygen vibration mode appears around 551 cm−1 in the FTIR spectra which grows in intensity with increased Ti-doping. The peak position of the CuO2 planar oxygen mode is not affected with the doping of Ti showing that Ti is doped at the in-chain-copper Cu(1) sites in the unit cell. The fluctuation induced conductivity analyses reveals that despite an increase in the parameters such as ξc, VF, and inter-layer coupling J, the critical temperature and superconductor volume fraction are suppressed. It shows that the doped Ti-atoms most likely induce anharmonic oscillations which may suppress the density of phonon population and hence the superconductivity. It can also be witnessed in the decreased phase relaxation time of the carriers with increasing Ti-doping. These studies stress on the essential role of soft phonons in the mechanism of high Tc superconductivity in YBa2(Cu3−xTix)O7−δ superconductors.

Hybrid crystals of cuprates and iron-based superconductors**Project supported by the National Basic Research Program of China (Grant No. 2015CB921300), the National Natural Science Foundation of China (Grant Nos. 1190020 and 11334012), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB07000000).

We propose two possible new compounds, Ba2CuO2Fe2As2 and K2CuO2Fe2Se2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO2 layers and antifluorite-type Fe2X2 (X = As, Se) layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high Tc superconductivity.

Gate-tuned superconductor-insulator transition in (Li,Fe)OHFeSe

The antiferromagnetic(AFM) insulator-superconductor transition has been always a center of interest in the underlying physics of unconventional superconductors. The quantum phase transition between Mott insulator with AFM and superconductor can be induced by doping charge carriers in high-Tc cuprate superconductors. For the best characterized organic superconductors of k-(BEDT-TTF)2X (X=anion), a first order transition between AFM insulator and superconductor can be tuned by applied external pressure or chemical pressure. Also, the superconducting state can be directly developed from AFM insulator by application of pressure in Cs3C60. The resemblance of these phase diagrams hints a universal mechanism governing the unconventional superconductivity in close proximity to AFM insulators. However, the superconductivity in iron-based high-Tc superconductors evolves from an AFM bad metal by doping charge carriers, and no superconductor-insulator transition has been observed so far. Here, we report a first-order transition from superconductor to insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe. The Tc is continuously enhanced with electron doping by ionic gating up to a maximum Tc of 43 K, and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest Tc. A novel phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out, indicating that the superconductor -insulator transition is a common feature for unconventional superconductivity. These results help to uncover the underlying physics of iron-based superconductivity as well as the universal mechanism of high-Tc superconductivity. Our finding also suggests that the gate-controlled strong charge doping makes it possible to explore novel states of matter in a way beyond traditional methods.

Electron–Phonon Interactions and Superconductivity in (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y )O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0)

We have used two-step solid state reaction method for the synthesis of (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y ) O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) superconductors at 880 °C. The oxygen contents in the samples were optimized by carrying out self-doping which is accomplished by post-annealed in flowing oxygen environment at 500 °C for approximately 5 h. The superconducting properties of all the samples were improved, after the self-doping of the carriers in the conducting planes. In the x-ray diffraction scans of the samples, the a-axis length of tetragonal unit cell increases whereas the c-axis decreases with increased Ti doping in the final compound. The Fourier transform infrared spectrometer (FTIR) absorption measurements of these samples have shown that the apical oxygen mode at 548 cm−1 and the planar oxygen mode at 596 cm−1 are softened with increased Ti doping. The origin of softening of planar oxygen mode lies in increase bond lengths of apical oxygen atoms promoted by larger covalent radius of Ti (1.32 A) atoms relative to Cu (1.17 A) atoms that in turn promotes the softening of the apical oxygen modes. Doped Ti (47.90 amu) atoms at the Cu (63.54 amu) sites initiate the an-harmonic oscillations resulting into the suppression of density of phonon modes. The suppression in the values of superconductivity parameters with Ti doping at the Cu sites shows the essential role of phonon in mechanism of high Tc superconductivity and hence the electron–phonons interactions. The excess conductivity analyses (FIC) of conductivity data of oxygen-post-annealed samples have shown decrease in the mean field critical temperature, coherence length along the c-axis, interlayer coupling, and Fermi velocity with increase Ti doping. However, the values of B c, B c1, and J c(0) increase with Ti doping, showing increase in the population of the pinning centers.

Pair–pair interactions as a mechanism for high-Tc superconductivity

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above Tc, pre-existing but fluctuating Cooper pairs give rise to the unconventional pseudogap (PG) state, well-characterized by experiment. At the critical temperature, the pair–pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below Tc, both the condensation energy and the pair–pair interaction β are proportional to the condensate density Noc(T), while the usual Fermi-level spectral gap is independent of temperature. The new order parameter , can be followed as a function of temperature, carrier concentration and disorder—i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the coupling of quasiparticles to Cooper-pair excitations. The latter interpretation is strongly supported by the observed quasiparticle spectral function.