Possibility Of Superconductivity Research Articles

Role of Lifshitz transition stabilized carbon hexagon structure from biaxial strain: A case of sodium carbide with superconducting condition

We theoretically report on an investigation of sodium carbide system by means of first-principles calculations based on density functional theory. Herein, the diverse sodium–carbon structures are predicted by taking carbon-rich compositions of NaC2 with a carbon hexagon structure as a starting point. Metallic phases of NaC2 at a pressure of 100GPa are predicted to be stabilized by biaxial strain, culminating in the strain-induced electronic topological transitions, also known as the Lifshitz transitions. We found that the flat band accommodates localized electrons around the Fermi level, originating from the effect of biaxial strain, which results in low-velocity electrons forming up to at least 20% of Cooper pairs. According to the respective phonon-mediated superconductivity, NaC2 is dynamically stable not only without the influence of biaxial strain but also with that of the biaxial tensile strain, indicating possible enhancement of the critical temperature superconductor (Tc). Furthermore, the estimated Tc reaches 29.5 K, slightly higher than 24.7 K for the case without biaxial strain. These findings suggest that the possibility of superconductivity is promoted by the applied biaxial tensile strain. Our findings pave the way for future investigation of high superconductivity in carbon-based materials and suggest the possibility of metal-carbides being synthesisable and exhibiting quite a high Tc superconductivity.

Pressure induced superconductivity in MnSe

The rich phenomena in the FeSe and related compounds have attracted great interests as it provides fertile material to gain further insight into the mechanism of high temperature superconductivity. A natural follow-up work was to look into the possibility of superconductivity in MnSe. We demonstrated in this work that high pressure can effectively suppress the complex magnetic characters of MnSe, and induce superconductivity with Tc ~ 5 K at pressure ~12 GPa confirmed by both magnetic and resistive measurements. The highest Tc is ~ 9 K (magnetic result) at ~35 GPa. Our observations suggest the observed superconductivity may closely relate to the pressure-induced structural change. However, the interface between the metallic and insulating boundaries may also play an important role to the pressure induced superconductivity in MnSe.

First-principles calculations of monolayer hexagonal boron nitride: Possibility of superconductivity

Based on density-functional first-principles calculations and the Wannier interpolation technique, we have investigated the electron-phonon coupling (EPC) in hole-doped monolayer hexagonal boron nitride (h-BN). We align the Fermi level to a Van Hove singularity by 0.4 holes/cell doping, the largest hole concentration that can be realized in monolayer h-BN. Under this doping level, the EPC for free-standing monolayer h-BN is weak. However, biaxial tensile strain can enhance the EPC significantly. We predict that monolayer h-BN can become a phonon-mediated superconductor by the synergic effects of hole doping and biaxial tensile strain, with transition temperature exceeding 40 K.

On the Possibility of Superconductivity in Bilayer Heterostructures

A model is created for bilayer heterostructures in a strong magnetic field which makes it possible to neglect the Coulomb interaction. The thermodynamic instability of states of the electron system in a strong magnetic field leads to the formation of a periodic vortex lattice. The case is considered where the electron density is close to the density of the half-filled Landau level. An electron spectrum is found and an analog of the Cooper effect appearing under the Bogoliubov canonical transformation for electron Fermi operators is studied.

On possibility of superconductivity in SnSb: A first principle study

The electronic, phonon structure and superconducting properties of tin antimonide (SnSb) in rock-salt (RS) structure are calculated using first-principles density functional theory. The electronic band structure and density of states show metallic behavior. The phonon frequencies are positive throughout the Brillouin zone in rock-salt structure indicating its stability in that phase. Superconductivity of SnSb in RS phase is discussed in detail by calculating phonon linewidths, Eliashberg spectral function, electron-phonon coupling constant and superconducting transition temperature. SnSb is found to have a slightly lower TC (3.1K), as compared to SnAs.

Magnetism and superconductivity in a quasi-2D anisotropic system doped with charge carriers

The theory of multiband superconducting systems with variable density of charge carriers is analyzed. The possibility of emergence of nonphonon high-temperature superconductivity due to the predominance of electron–electron interband interactions over intraband interactions, as well as due to the fact that the thermodynamic and magnetic properties of multiband systems in the superconducting phase differ qualitatively from those of single-band systems, is indicated. Phase transitions in a quasi-2D anisotropic medium upon a change in the carrier concentration, i.e., a transition from the commensurate to the incommensurate state of the spin density wave, are analyzed. Such a transition is observed when the Umklapp processes in the lattice structure are taken into account. These processes facilitate a deviation of wavevector Q of the spin density wave from 2k F , as well as a displacement of the bandgap relative to the Fermi surface. This leads to the generation of free charge carriers and the possibility of superconductivity. It is shown that superconductivity accompanies the magnetism. The conditions for the coexistence of these two phenomena are determined.

Superconductivity in Ca-doped graphene laminates.

Despite graphene’s long list of exceptional electronic properties and many theoretical predictions regarding the possibility of superconductivity in graphene, its direct and unambiguous experimental observation has not been achieved. We searched for superconductivity in weakly interacting, metal decorated graphene crystals assembled into so-called graphene laminates, consisting of well separated and electronically decoupled graphene crystallites. We report robust superconductivity in all Ca-doped graphene laminates. They become superconducting at temperatures (Tc) between ≈4 and ≈6 K, with Tc’s strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration in graphene. We find that Ca is the only dopant that induces superconductivity in graphene laminates above 1.8 K among several dopants used in our experiments, such as potassium, caesium and lithium. By revealing the tunability of the superconducting response through doping and confinement of the metal layer, our work shows that achieving superconductivity in free-standing, metal decorated monolayer graphene is conditional on an optimum confinement of the metal layer and sufficient doping, thereby bringing its experimental realization within grasp.

First-principle investigations of structure, elastic and bond hardness of FexB (x=1, 2, 3) under pressure

First-principles calculations are performed to study pressure effects on structure, magnetic and mechanical properties of FexB (x=1, 2, 3) compounds using density functional theory (DFT) within GGA approximation. The three structures are studied in the pressure range from 0 to 90GPa, in order to predict the critical transition pressure from magnetic to nonmagnetic states (NM) and a possibility of superconductivity in this state was predicted. The density of states of FexB ferromagnetic compounds are significantly modified with increasing pressure and at particular critical pressures, our compounds undergo an abrupt loss of ferromagnetic character that cause mirror in upper and lowers half panels on both spin channels. Furthermore, the relationship between crystal structure and material hardness of FexB is also investigated by calculating hardness of Fe–B and B–B bonds using Mulikan population analysis and semi empirical hardness theory. This model proved effective in hardness prediction of metal borides and agrees well with the experimental values. By the elastic stability criteria, it is predicted that FexB are stable up to the selected pressures.

Magnetic Properties and the Superatom Character of 13-Atom Platinum Nanoclusters

13-atom platinum nanoclusters have been synthesized quantitatively in the pores of the zeolites NaY and KL. They reveal highly interesting magnetic properties like high-spin states, a blocking temperature, and super-diamagnetism, depending heavily on the loading of chemisorbed hydrogen. Additionally, EPR active states are observed. All of these magnetic properties are understood best if one considers the near-spherical clusters as analogs of transition metal atoms with low-spin and high-spin states, and with delocalized molecular orbitals which have a structure similar to that of atomic orbitals. These clusters are, therefore, called superatoms, and it is their analogy with normal atoms which is in the focus of the present work, but further phenomena, like the observation of a magnetic blocking temperature and the possibility of superconductivity, are discussed.

Chiral triplet superconductivity on the graphene lattice

Motivated by the possibility of superconductivity in doped graphene sheets, we investigate superconducting order in the extended Hubbard model on the two-dimensional graphene lattice using the variational cluster approximation (VCA) and the cellular dynamical mean-field theory (CDMFT) with an exact diagonalization solver at zero temperature. The nearest-neighbor interaction is treated using a mean-field decoupling between clusters. We compare different pairing symmetries, singlet and triplet, based on short-range pairing. VCA simulations show that the real (nonchiral), triplet $p$-wave symmetry is favored for small $V$, small onsite interaction $U$, or large doping, whereas the chiral combination $p+ip$ is favored for larger values of $V$, stronger onsite interaction $U$, or smaller doping. CDMFT simulations confirm the stability of the $p+ip$ solution, even at half-filling. Singlet superconductivity (extended $s$ wave or $d$ wave) is either absent or subdominant.

Observation of Superconductivity in Tetragonal FeS.

The possibility of superconductivity in tetragonal FeS has attracted considerable interest because of its similarities to the FeSe superconductor. However, all efforts made to pursue superconductivity in tetragonal FeS have failed so far, and it remains controversial whether tetragonal FeS is metallic or semiconducting. Here we report the observation of superconductivity at 5 K in tetragonal FeS that is synthesized by the hydrothermal reaction of iron powder with sulfide solution. The obtained samples are highly crystalline and less air-sensitive, in contrast to those reported in the literature, which are meta-stable and air-sensitive. Magnetic and electrical properties measurements show that the samples behave as a paramagnetic metal in the normal state and exhibit superconductivity below 5 K. The high crystallinity and the stoichiometry of the samples play important roles in the observation of superconductivity. The present results demonstrate that tetragonal FeS is a promising new platform to realize high-temperature superconductors.

One-dimensional alkali-dopedC60chains encapsulated in BN nanotubes

We study the energetics, electronic structures, and electron-phonon couplings of a one-dimensional potassium-doped ${\mathrm{C}}_{60}$ chain encapsulated in a boron nitride nanotube using the framework of the density-functional theory. We demonstrate that the reaction of potassium doping is exothermic and the resulting material is one-dimensional metal where conducting electrons are only in the ${\mathrm{C}}_{60}$ chain. Interestingly, the Fermi-level density of states has a peculiar pressure dependence and can be larger than those in the three-dimensional alkali-doped fullerene compounds, indicating the possibility of various phase transitions. We also discuss the electron-phonon couplings and the possibility of superconductivity.

Emergence of non-equilibrium superconductivity originated from repulsive interaction: Demonstration using optical lattices and implication to solid-state matter system

We study the dynamical properties of ultracold fermions in one-dimensional optical superlattices by using the adaptive time-dependent density matrix renormalization group method. The system is repulsive Hubbard model with an two-site periodic superlattice potential. Owing to superlattice structure, the ground-state states become the Mott-type insulating state at quarter-filling and band-type insulating state at half-filling, respectively. We clarify the dynamical properties of time evolution when the system is non-adiabatically changed to another lattice structure (i.e., the superlattice potential is suddenly changed to a normal one). In the case of Mott-type insulating state at quarter-filling, the time evolution exhibits a profile similar to that expected for single atom. On the other hand, we clarify the dynamical properties of a band-type insulating state at half-filling. The strongly-correlated interaction an unusual pairing of fermions induced the pair hopping process. We further address the robustness of pair hopping process and possibility of superconductivity by using sudden change from superlattice structure to normal one.

Frustrated Metallic Systems: A Review of Some Peculiar Behavior

While consequences of frustration of magnetic interactions are much studied in localized spin systems, less studies have been performed on frustrated metallic systems. In this review I show that several effects due to strong frustration have also been observed in some metallic correlated systems compounds containing rare-earth or transition metal magnetic atoms. There are also effects, specific to frustrated metallic systems, which were theoretically predicted but not yet observed. This paper will review some of these aspects, concerning either the magnetically ordered systems, as the existence of mixed magnetic structures, or the anomalous Hall effect in chiral magnetic structures, or in non ordered systems, as the heavy fermion behavior or the possibility of superconductivity. The last part is devoted to the metal–insulator transition in these systems.

Pressure-induced Quantum Phase Transition in Cr1-xVx Alloys

The electrical resistivities and Hall effect of Cr1-xVx alloys(xN of x = 0.015, 0.032 and 0.035 was found to disappear at corresponding Pc of 5.6 GPa, 1.1 GPa and 0.7 GPa, respectively. The Hall coefficients of x = 0.032 and 0.035 demonstrated a discontinuous change around Pc, which implies a quantum phase transition induced by high pressure. We found that the exponents in the power law of resistivity of x = 0.015 had an anomaly around Pc, which means that the large quantum fluctuations affect the resistivity even at high temperature. A possibility of superconductivity is explored near Pc for x = 0.032. A slight decrease in the ρ(T) curve was observed below 1 K at high pressure. The origins for that are briefly discussed.

Electronic structures and three-dimensional effects of boron-doped carbon nanotubes

We study boron-doped carbon nanotubes by first-principles methods based on the density functional theory. To discuss the possibility of superconductivity, we calculate the electronic band structure and the density of states (DOS) of boron-doped (10,0) nanotubes by changing the boron density. It is found that the Fermi level density of states D(∊F) increases upon lowering the boron density. This can be understood in terms of the rigid band picture where the one-dimensional van Hove singularity lies at the edge of the valence band in the DOS of the pristine nanotube. The effect of three-dimensionality is also considered by performing the calculations for bundled (10,0) nanotubes and boron-doped double-walled carbon nanotubes (10,0)@(19,0). From the calculation of the bundled nanotubes, it is found that interwall dispersion is sufficiently large to broaden the peaks of the van Hove singularity in the DOS. Thus, to achieve the high D(∊F) using the bundle of nanotubes with single chirality, we should take into account the distance from each nanotube. In the case of double-walled carbon nanotubes, we find that the holes introduced to the inner tube by boron doping spread also on the outer tube, while the band structure of each tube remains almost unchanged.

On Possibility of Superconductivity in Thin Films

Abstract Behavior of electrons inside a thin coating layer is studied. It is based on Cheng-Born theory of broken symmetry and TFDC(Thomas-Fermi-Dirac-Cheng) electron theory. For a specimen (TiO 2 , say) coated on a conducting metal plate (e.g. platinum), according to the TFDC model, a small group of electrons (or holes) will transfer from one side of boundary to the other. Based on Cheng-Born theory, in case only few electrons exist in zone, only a few corners are occupied, broken symmetry in momentum space will occur and lead to advent of superconductivity. Following the general formulation of thermodynamics, an estimate on the transition temperature in two-dimensional space is made. The result shows that the superconductors in two-dimensions are in general inherent to HTS, T c of superconductor in two-dimensions would be at least by one order larger than T c of normal superconductor in three-dimensions. In this paper an experiment is designed to look for superconductivity of thin film.

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15–δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr2Ba4Cu7O15−δ (Pr247) showing a superconducting transition at Tc=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T2 dependence up to approximately 150 K. A clear specific heat anomaly was observed at Tc for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature Tn of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μB. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit Tα dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.PACS: 74.72.Jt; 74.25.Bt; 75.40.Cx; 74.25.Fy

Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

Using density functional theory we investigate the occurrence of superconductivity in AC$_6$ with A=Mg,Ca,Sr,Ba. We predict that at zero pressure, Ba and Sr should be superconducting with critical temperatures (T$_c$) 0.2 K and 3.0 K, respectively. We study the pressure dependence of T$_c$ assuming the same symmetry for the crystal structures at zero and finite pressures. We find that the SrC$_6$ and BaC$_6$ critical temperatures should be substantially enhanced by pressure. On the contrary, for CaC$_6$ we find that in the 0 to 5 GPa region, T$_c$ weakly increases with pressure. The increase is much smaller than what shown in several recent experiments. Thus we suggest that in CaC$_6$ a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although MgC$_6$ is unstable, the synthesis of intercalated systems of the kind Mg$_x$Ca$_{1-x}$C$_y$ could lead to higher critical temperatures.

Possible clean superconductivity in doped nanotube crystals

Abstract We discuss the possibility of superconductivity in the “clean limit” in superconductors made of a doped superlattice of nanotubes. The lattice architecture determines the subbands of the electronic structure. The disparity and different spatial locations of the wave functions of electrons in different subbands at the Fermi level suppress the single-electron impurity interband scattering giving multiband superconductivity in the clean limit. The Feshbach shape resonance increases the critical temperature by tuning the chemical potential at the Lifshitz electronic topological transition where the Fermi surface of one of the bands changes from the one-dimensional (1D) to the two-dimensional (2D) topology.