high-Tc Materials Research Articles

Prediction of ambient pressure superconductivity in cubic ternary hydrides with MH6 octahedra

Exploring high-temperature superconducting (high-Tc) material at ambient pressure holds immense significance for physics, chemistry, and materials science. In this study, we perform a high-throughput screening of strong electron-phonon interactions in X2MH6 compounds (X = Li, Na, Mg, Al, K, Ca, Ga, Rb, Sr, and In; M are 3d, 4d, and 5d transition metals). These compounds have a cubic structure featuring an MH6 octahedron motif. Our screening calculations suggest that 26 compounds exhibit dynamic stability and strong electron-phonon coupling. Among them, Mg2RhH6, Mg2IrH6, Al2MnH6, and Li2CuH6 show promising energetic stability and Tc of more than 50 K at ambient pressure. This study underscores promising high-Tc compounds at ambient pressure with distinctive MH6 motifs.

Research and mechanism of iron-based superconductors

The discovery of new iron-based superconductor LaFeAsO1-xFx with Tc of 26 K in 2008 had quickly revived interest in the exploration of iron-based superconductors. After several years of research on this superconducting material, iron-based superconductors have become another family of high-temperature superconductors after copper-based superconductors. The crystal structure of LaOFeAs is P4/nmm family, and by doping with fluorine, LaO1-xFxFeAs exhibits superconductivity because both structural distortion and magnetic order are suppressed. In this review, the development and experimental results of such iron-based high Tc materials will be presented. We describe the synthetic method of this class of superconductors, the physical properties of the precursors, the doping methods that can produce superconductivity, the pressure effect, and the prospects of this new iron-based high Tc family.

Machine learning predictions of high-Curie-temperature materials

Technologies that function at room temperature often require magnets with a high Curie temperature, TC, and can be improved with better materials. Discovering magnetic materials with a substantial TC is challenging because of the large number of candidates and the cost of fabricating and testing them. Using the two largest known datasets of experimental Curie temperatures, we develop machine-learning models to make rapid TC predictions solely based on the chemical composition of a material. We train a random-forest model and a k-NN one and predict on an initial dataset of over 2500 materials and then validate the model on a new dataset containing over 3000 entries. The accuracy is compared for multiple compounds' representations (“descriptors”) and regression approaches. A random-forest model provides the most accurate predictions and is not improved by dimensionality reduction or by using more complex descriptors based on atomic properties. A random-forest model trained on a combination of both datasets shows that cobalt-rich and iron-rich materials have the highest Curie temperatures for all binary and ternary compounds. An analysis of the model reveals systematic error that causes the model to over-predict low-TC materials and under-predict high-TC materials. For exhaustive searches to find new high-TC materials, analysis of the learning rate suggests either that much more data is needed or that more efficient descriptors are necessary.

Terahertz Absorption Spectroscopy in Bicrystal Josephson Junctions Formed from Mutually Tilted c-Axes YBa2Cu3O7−x Films

Spectral analysis of terahertz (THz) and sub-THz emission from quantum cascade lasers has been recently demonstrated using conventional YBa2Cu3O7−x bicrystal Josephson junctions made from c-axes thin films. Josephson frequencies of alternative bicrystal junctions made from YBa2Cu3O7−x films with mutually tilted c-axes extend further into the THz range. However, these THz oscillations can weaken due to new absorption channels in the junction environment. Here, using Josephson admittance spectroscopy, THz losses in YBa2Cu3O7−x bicrystal junctions with mutually tilted c-axes are studied. Absorption maximizes at a reproducible set of THz frequencies close to those of collective modes in bulk YBa2Cu3O7−x recovered by Fourier spectroscopy. Annealing junctions in atomic oxygen reduces the losses at frequencies of 2.7 and 3.6 THz, while the losses increase at frequencies of 2.3 and 4.6 THz. Thus, as a THz spectrum analyzer, YBa2Cu3O7−x bicrystal junctions require post-fabrication correction of the oxygen content. In addition, the fine structure of the absorption spectrum appears at frequencies near 4.6 THz. Significant absorption near 2.3 THz may be due to effects associated with the second Josephson harmonic or second-order nonlinearity of the susceptibility in YBa2Cu3O7−x. This work paves the way towards probing collective modes in high-Tc materials in situ using the Josephson oscillations.

Some finite temperature and ground state properties of high-Tc superconductivity within the planar t-J-U model

In high-Tc materials, considering pair hopping and on-site pairing of electrons, presence of two electrons at a site following Pauli exclusion principle (a t-J-U model) is relevant. Considering this novelty, within a two-dimensional (2D) t-J-U Hamiltonian, some thermodynamic and ground state properties of high-Tc cuprates at various hole doping concentrations (〈h〉) have been investigated. Any theory based on perturbation approximation is inapplicable in strongly correlated electron systems, therefore, Lanczos method of exact diagonalization on a square cluster (tilted) of 8-sites is followed. Results show that double occupancy (δ) is maximum for hole concentration 〈h〉 = 0.25 and increases at a particular temperature T = T(U). Local moment shows the opposite behavior. Spin-spin correlation is negative between nearest-neighbor sites, confirming antiferromagnetic nature of the system. It appears that on-site Coulomb repulsion (U/t ≥ 6.0) destabilizes the superconducting ground state. In the system, antiferromagnetism (AF) and superconductivity are seen to coexist. A uniform distribution of charges is seen with increasing on-site Coulomb repulsion (U), which results in constant electron–electron correlation.

Two distinctive temperature dependences of the London penetration depth in high-Tc cuprate superconductors as support for the theory of Bose-liquid superconductivity

In the framework of the theory of Bose-liquid superconductivity, we show that in high-Tc cuprate superconductors the magnetic field penetration depth (known as the London penetration depth λL(T)) decreases first exponentially below the superconducting transition temperature Tc with decreasing the temperature T and then λL(T) decreases like a certain power of T at lower temperatures due to the vanishing of the energy gap Δg(T) in the excitation spectrum of a superfluid Bose-liquid at a characteristic temperature Tc⁎ lower than Tc. Below Tc, the London penetration depth λL(T) has an exponential temperature dependence down to Tc⁎, which is different from a BCS- like exponential temperature dependence, and then λL(T) will follow a power law temperature dependence down to T=0, which is also different from power-law temperature dependences predicted by some BCS-like (d-wave) pairing theories. In unconventional (bosonic) cuprate superconductors characterized by the strong interboson coupling constant (γB≳1), λL(T) exhibits the distinctly different temperature dependences in different temperature intervals 0≤T≤Tc⁎<<Tc and Tc⁎<T<Tc when the energy gap Δg(T) vanishes at a temperature Tc⁎ well below Tc. In the weak interboson coupling (γB<<1) the energy gap Δg(T) vanishes at a temperature Tc⁎ close to Tc and the reduced London penetration depth λL(T)/λL(0) has an unusual Tn power-law temperature dependence in a wide temperature range 0<T<Tc⁎ below Tc, i.e. the expression λL(T)/λL(0)≃[1−aL(T)(T/Tc)n]−1/2 (where n≳4 and aL(T)≲1) is valid only below Tc⁎ and is similar to the well-known empirical Gorter-Casimir law. Our theoretical predictions of the distinctive power-law and exponential temperature dependences of λL(T)/λL(0) are in good agreement with the experimental results reported for GdBa2Cu3O7−δ ceramics and YBa2Cu3O7−δ films. The peculiar temperature dependences of λL(T)/λL(0) observed in these high-Tc materials at low and high temperatures do not follow the predictions of the s- and d-wave BCS-like theories of Fermi-liquid superconductivity and give evidence for the predictions of the theory of Bose-liquid superconductivity.

Orbital selective hole- and hole-pair formation and Bose condensation in high-temperature superconductors

Miguel Alario-Franco is recognised not only for his seminal experimental advances in the field of High Tc superconductivity but also for his incisive observations on the advances and challenges in that field. In 1995, he highlighted the need for “a new theme” to unite the chemistry and the physics of this spectacular natural phenomenon following the advance by Bednorz and Muller, that surely ranking as one of the greatest-ever discoveries in science. We review the remarkable recent advances that showed hole-pairing in the cuprates is found at temperatures above Tc; first proposed over three decades ago by Mott and Alexandrov. We highlight that the pairing itself will be ‘orbital-selective’ where holes of predominantly one specific orbital character bind to form the Cooper pairs which then condense to yield the High Tc state at a critical density and temperature. Here, a collection of friends and colleagues of Miguel from the disciplines of chemistry, physics and electronic engineering outline this as a working, chemically-intuitive model in the search for other High Tc materials.

Thermodynamics of Point Defects in Solids and Relation with the Bulk Properties: Recent Results

For several decades, the crucial question has arisen as to whether there exists any direct interconnection between the thermodynamic parameters of point defects in solids with the bulk properties of the solid under investigation. To answer this important question, an interrelation of the defect Gibbs energy gi in solids with bulk properties has been proposed almost half a century ago. Considering that gi corresponds to an isobaric and isothermal process, this interrelation states that, for different processes (defect formation, self-diffusion activation, and heterodiffusion), gi is proportional to the isothermal bulk modulus B and the mean volume per atom Ω, termed cBΩ model. Here, we review several challenging applications of this interrelation that appeared during the last decade (2011–2021), including high pressure diamond anvil measurements, high Tc superconductors, nuclear fuels, and materials for micro-electronics devices, applications of usefulness in Geophysics and Seismology, a problem of major technological interest, search for compositions of better target properties in Cu-Co-Si alloys via machine learning as well as two independent studies on the physical origin of this interrelation that has been further strengthened during the last few years.

Microscopic Theory of Novel Pseudogap Phenomena and Bose-Liquid Superconductivity and Superfluidity in High-Tc Materials and Other Systems

Microscopic Theory of Novel Pseudogap Phenomena and Bose-Liquid Superconductivity and Superfluidity in High-Tc Materials and Other Systems

Phonons, electrons and thermal transport in Planckian high Tc materials

The room-temperature thermal diffusivity of high Tc materials is dominated by phonons. This allows the scattering of phonons by electrons to be discerned. We argue that the measured strength of this scattering suggests a converse Planckian scattering of electrons by phonons across the room-temperature phase diagram of these materials. Consistent with this conclusion, the temperature derivative of the resistivity of strongly overdoped cuprates is noted to show a kink at a little below 200 K that we argue should be understood as the onset of a high-temperature Planckian T-linear scattering of electrons by classical phonons. This kink continuously disappears toward optimal doping, even while strong scattering of phonons by electrons remains visible in the thermal diffusivity, sharpening the long-standing puzzle of the lack of a feature in the T-linear resistivity at optimal doping associated with the onset of phonon scattering.

Unusual metallic conductivity of high-Tc cuprates

The intrinsic mechanisms of the unusual metallic transports of three types of relevant charge carriers (large polarons, excited (dissociated) polaronic components of bosonic Cooper pairs and bosonic Cooper pairs themselves) along the CuO2 layers of high-Tc cuprates are identified and the new features of metallic conductivity in the CuO2 layers (i.e. ab -planes) of underdoped and optimally doped cuprates are explained. The in-plane conductivity of high-Tc cuprates is associated with the metallic transports of such charge carriers at their scattering by lattice vibrations in thin CuO2 layers. The proposed charge transport theory in high-Tc cuprates allows to explain consistently the distinctive features of metallic conductivity and the puzzling experimental data on the temperature dependences of their in-plane resistivity pab. In underdoped and optimally doped cuprates the linear temperature dependence of pab(T) above the pseudogap formation temperature T∗ is associated with the scattering of polaronic carriers at acoustic and optical phonons, while the different (upward and downward) deviations from the linearity in pab(T) below T∗ are caused by the pseudogap effect on the conductivity of the excited Fermi components of bosonic Cooper pairs and by the dominating conductivity of bosonic Cooper pairs themselves in the normal state of these high-Tc materials.

Effect of metal atoms on the superconducting properties of decahydrides, MH10 (M = Ga, Nb & Mo)

Applying first principles calculations we have explored the effect of metal atoms on the superconducting properties of decahydrides of Gallium (Ga), Niobium (Nb) and Molybdenum (Mo) at 150 GPa. These atoms owing to their apt electronegativities, metallic radii and other physical parameters stabilise the host guest type clathrate structure of decahydrides. Our calculations show the highest density of state (DOS) (N(EF) = 2.62 states/eV/f.u.) at the Fermi level for NbH10 along with presence of van Hove singularity among all decahydrides. This N(EF) value is ~163% enhanced compared to that of LaH10 at 150 GPa making it a sought for high TC material. Our correlative analysis establishes a pivotal role for chemical properties, in particular, electronegativity of metal atoms in tuning the H–H distances and hence the superconductivity of decahydrides.

Electrochemical Synthesis and Crystal Structure of the Organic Ion Intercalated Superconductor (TMA)0.5Fe2Se2 with Tc = 43 K.

Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using β-FeSe crystals (Tc ≈ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first-principle density functional theory calculations constrain the specified structure. Our synthetic route enables electrochemical intercalations of other organic cations with high yields to greatly optimize the superconducting properties and to expand this class of high-Tc materials.

High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure.

The discovery of H3S and LaH10 is an important step towards the development of room temperature superconductors which fuels the enthusiasm for finding promising superconductors among hydrides at high pressure. In the present study, three new and stable stoichiometric MoH5, MoH6 and MoH11 compounds were found in the pressure range of 100-300 GPa. The highly hydrogen-rich phase of Cmmm-MoH11 has a layered structure that contains various forms of hydrogen: H, H2- and H3- units. It is a high-Tc material with an estimated Tc value in the range of 165-182 K at 250 GPa. The same structures are also found in NbH11, TaH11, and WH11, each material showing Tc ranging from 117 to 168 K. By combining the method of using two coupling constants λopt and λac, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of Tc are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.

High-Tc Cooper-pair injection in a semiconductor–superconductor structure

We observe Andreev reflection in a YBCO–GaN junction through differential conductance spectroscopy. A strong characteristic zero-bias peak was observed and persisted up to the critical temperature of the superconductor with a smaller superconducting order parameter Δ ∼ 1 meV. The presence of Andreev reflection with the small Δ in comparison to its value for high-Tc superconductors forms an important milestone toward demonstration of superconducting proximity in high-Tc/semiconductor junctions. Experimental results were then compared to the theoretical model with good agreement. Efficient injection of Cooper pairs into direct bandgap semiconducting structures, together with high transition temperature of YBCO, can pave the way to novel optoelectronics and quantum optical studies of high-Tc materials.

Frustration-induced supersolid phases of extended Bose–Hubbard model in the hard-core limit

We investigate exotic supersolid phases in the extended Bose–Hubbard model with infinite projected entangled-pair state, numerical exact diagonalization, and mean-field theory. We demonstrate that many different supersolid phases can be generated by changing signs of hopping terms, and the interactions along with the frustration of hopping terms are important to stabilize those supersolid states. We argue the effect of frustration introduced by the competition of hopping terms in the supersolid phases from the mean-field point of view. This helps to give a clearer picture of the background mechanism for underlying superfluid/supersolid states to be formed. With this knowledge, we predict and realize the d-wave superfluid, which shares the same pairing symmetry with high-Tc materials, and its extended phases. We believe that our results contribute to preliminary understanding for desired target phases in the real-world experimental systems.

Superconductivity in La and Y hydrides: Remaining questions to experiment and theory

Recent reports of the superconductivity in hydrides of two different families (covalent lattice, as in SH3 and clathrate-type H-cages containing La and Y atoms, as in LaH10 and YH6) have revealed new families of high-Tc materials with Tc’s near room temperature values. These findings confirm earlier expectations that hydrides may have very high Tc’s due to the fact that light H atoms have very high vibrational frequencies, leading to high Tc values within the conventional Bardeen–Cooper–Schrieffer phonon mechanism of superconductivity. However, as is pointed out by Ashcroft, it is important to have the metallic hydrogen “alloyed” with the elements added to it. This concept of a metallic alloy containing a high concentration of metal-like hydrogen atoms has been instrumental in finding new high-Tc superhydrides. These new superhydride “room-temperature” superconductors are stabilized only at very high pressures above 100 GPa, making the experimental search for their superconducting properties very difficult. We will review the current experimental and theoretical results for LaH10−x and YH6−x superhydrides.

Structure of the Order Parameter of Topological Superconductors Based on 3d and 4d Transition Metal Oxides

We consider experimental data on the order parameter structure in superconductors with a layered perovskite structure: La2CuO4 and Sr2RuO4. Experimental data are analyzed using the space group approach to two-electron states in crystals. From the crystalline symmetry of Sr2RuO4, we derive two-electron functions with Eu symmetry, having nodal planes consistent with experimental data. In the case of La2CuO4 and high-Tc materials, the order parameter corresponds to the irreducible representation B1g with hidden symmetry D4h. It is also shown that the charge density waves found in pseudogap states of high-Tc materials can be thought of as two-electron states of equivalent electrons with a nonzero momentum.

Universal superconducting precursor in three classes of unconventional superconductors

A pivotal challenge posed by unconventional superconductors is to unravel how superconductivity emerges upon cooling from the generally complex normal state. Here, we use nonlinear magnetic response, a probe that is uniquely sensitive to the superconducting precursor, to uncover remarkable universal behaviour in three distinct classes of oxide superconductors: strontium titanate, strontium ruthenate, and the cuprate high-Tc materials. We find unusual exponential temperature dependence of the diamagnetic response above the transition temperature Tc, with a characteristic temperature scale that strongly varies with Tc. We correlate this scale with the sensitivity of Tc to local stress and show that it is influenced by intentionally-induced structural disorder. The universal behaviour is therefore caused by intrinsic, self-organized structural inhomogeneity, inherent to the oxides’ perovskite-based structure. The prevalence of such inhomogeneity has far-reaching implications for the interpretation of electronic properties of perovskite-related oxides in general.

Evidence of cooperative effect on the enhanced superconducting transition temperature at the FeSe/SrTiO3 interface

At the interface between monolayer FeSe films and SrTiO3 substrates the superconducting transition temperature (Tc) is unexpectedly high, triggering a surge of excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for seeking out higher Tc materials. To reveal this enigmatic mechanism, by combining advances in high quality interface growth, 16O leftrightarrow18O isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/SrTiO3 is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electrons and SrTiO3 phonons. Furthermore, our results point to the promising prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors.