Phonon-mediated Superconductivity Research Articles

Ground state, phonon spectrum, and superconducting properties of the cubic inverse perovskite CuNNi3

Using a first-principles approach, based on pseudopotentials and the density functional theory, we have investigated the origin of superconductivity in the cubic inverse perovskite CuNNi3. The electronic results reveal that the states around the Fermi level are mainly derived from Ni d orbitals. The average electron phonon coupling constant and the logarithmically averaged frequency are found to be 0.678 and 165.53K, respectively. The superconducting transition temperature is estimated as 3.34K, in good agreement with the experimentally reported value of 3.2K. We thus conclude that this material is a conventional phonon-mediated superconductor.

Study of the superconducting phase in silicene under biaxial tensile strain

The electron-doped silicene under the influence of the biaxial tensile strain is predicted to be the phonon-mediated superconductor. By using the Eliashberg formalism, we investigate the thermodynamic properties of the superconducting silicene in the case when the tension is 5% and the electron doping equals 3.5×1014cm−2. Under such conditions, silicene monolayer is expected to exhibit the highest superconducting transition temperature (TC). In particular, based on the electron–phonon spectral function and assuming a wide range of the Coulomb pseudopotential values (μ⋆∈〈0.1,0.3〉) it is stated that the superconducting transition temperature decreases from 18.7K to 11.6K. Similar behavior is observed in the case of the zeroth temperature superconducting energy gap at the Fermi level: 2Δ(0)∈〈6.68,3.88〉 meV. Other thermodynamic parameters differ from the predictions of the Bardeen–Cooper–Schrieffer theory. In particular, the ratio of the energy gap to the critical temperature changes in the range from 4.14 to 3.87. The ratio of the specific heat jump to the specific heat in the normal state takes the values from 2.19 to 2.05, and the ratio of the critical temperature and specific heat in the normal state to the thermodynamic critical field increases from 0.143 to 0.155. It is also determined that the maximum value of the electron effective mass equals 2.11 of the electron band mass.

Discovery of a superconducting high-entropy alloy.

High-entropy alloys (HEAs) are multicomponent mixtures of elements in similar concentrations, where the high entropy of mixing can stabilize disordered solid-solution phases with simple structures like a body-centered cubic or a face-centered cubic, in competition with ordered crystalline intermetallic phases. We have synthesized an HEA with the composition Ta34Nb33Hf8Zr14Ti11 (in at. %), which possesses an average body-centered cubic structure of lattice parameter a=3.36 Å. The measurements of the electrical resistivity, the magnetization and magnetic susceptibility, and the specific heat revealed that the Ta34Nb33Hf8Zr14Ti11 HEA is a type II superconductor with a transition temperature Tc≈7.3 K, an upper critical field μ0H_c2≈8.2 T, a lower critical field μ0Hc1≈32 mT, and an energy gap in the electronic density of states (DOS) at the Fermi level of 2Δ≈2.2 meV. The investigated HEA is close to a BCS-type phonon-mediated superconductor in the weak electron-phonon coupling limit, classifying it as a "dirty" superconductor. We show that the lattice degrees of freedom obey Vegard's rule of mixtures, indicating completely random mixing of the elements on the HEA lattice, whereas the electronic degrees of freedom do not obey this rule even approximately so that the electronic properties of a HEA are not a "cocktail" of properties of the constituent elements. The formation of a superconducting gap contributes to the electronic stabilization of the HEA state at low temperatures, where the entropic stabilization is ineffective, but the electronic energy gain due to the superconducting transition is too small for the global stabilization of the disordered state, which remains metastable.

Phonon-mediated superconductivity in compressed NbH4 compound

There has been much interest in the superconducting properties of metal hydrides for the last few years. The hydrogen-rich compounds are regarded as a way of hydrogen metallization at lower pressures than that required for pure hydrogen. This paper deals with the thermodynamic properties of the superconducting state in niobium hydride (NbH4) at 300 GPa. All theoretical calculations have been conducted within the framework of the strong-coupling Eliashberg theory. It has been stated that the critical temperature (T C ) is equal to 49.57 K when the Coulomb pseudopotential takes the commonly accepted value of 0.1. In the considered case, the ratio of the energy gap to the critical temperature (RΔ ≡ 2Δ(0)/k B T C), the ratio of the specific heat jump to the heat of the normal state (RC ≡ ΔC(T C)/C N(T C)) and the ratio connected with the thermodynamic critical field (R H ≡ T C C N(T C )/H 2 C(0)) are equal to 3.91, 2.12 and 0.154, respectively. Above results differ significantly from the values predicted by the weak-coupling Bardeen-Cooper-Schrieffer theory, which omits a very important role of the strong-coupling and phonon retardation effects.

Raman scattering investigation of the electron-phonon coupling in superconducting Nd(O,F)BiS2

We have performed polarized Raman scattering measurements on the newly discovered superconductor Nd(O,F)BiS$_2$ ($T_c = 4$ K). We observe 2 Raman active modes, with frequencies in accordance with first-principles calculations. One A$_{1g}$ phonon mode at 112.4 cm$^{-1}$ exhibits a Fano line shape due to electron-phonon coupling. We find a resonance for this mode at 2.45 eV excitation energy. We estimate a 0.68 contribution of this mode to the electron-phonon coupling constant $\lambda$. Our Raman results suggest that the BiS$_2$-based superconductors are possibly phonon-mediated BCS superconductors.

High-pressure effects in anti-post-perovskite superconductorsV3PnNx(Pn= P, As)

Pressure effects on the superconductors ${\mathrm{V}}_{3}$Pn${\mathrm{N}}_{x}$ (Pn = P and As) with the two-dimensional anti-post-perovskite structure were investigated. It was found that with increasing pressure, the superconducting transition temperature (${T}_{c}$) decreases with a negative differential pressure ($P$) coefficient $d{T}_{c}$/dP = \ensuremath{-}0.19 and \ensuremath{-}0.013 K/GPa for Pn = P and As, respectively. This is in marked contrast to the positive differential pressure coefficients reported for other vanadium-based superconductors with a three-dimensional crystal structure. The observed change in ${T}_{c}$ under pressure is revealed to be closely correlated with the evolution of the normal-state resistivity. The results support the phonon-mediated superconductivity in ${\mathrm{V}}_{3}$Pn${\mathrm{N}}_{x}$.

Observation of a universal donor-dependent vibrational mode in graphene

Electron-phonon coupling and the emergence of superconductivity in intercalated graphite have been studied extensively. Yet, phonon-mediated superconductivity has never been observed in the 2D equivalent of these materials, doped monolayer graphene. Here we perform angle-resolved photoemission spectroscopy to try to find an electron donor for graphene that is capable of inducing strong electron-phonon coupling and superconductivity. We examine the electron donor species Cs, Rb, K, Na, Li, Ca and for each we determine the full electronic band structure, the Eliashberg function and the superconducting critical temperature Tc from the spectral function. An unexpected low-energy peak appears for all dopants with an energy and intensity that depend on the dopant atom. We show that this peak is the result of a dopant-related vibration. The low energy and high intensity of this peak are crucially important for achieving superconductivity, with Ca being the most promising candidate for realizing superconductivity in graphene.

BaC: a thermodynamically stable layered superconductor.

To predict all stable compounds in the Ba-C system, we perform a comprehensive study using first-principles variable-composition evolutionary algorithm USPEX. We find that at 0 K the well-known compound BaC2 is metastable in the whole pressure range 0-40 GPa, while intercalated graphite phase BaC6 is stable at 0-19 GPa. A hitherto unknown layered orthorhombic Pbam phase of BaC has structure consisting of alternating layers of Ba atoms and layers of stoichiometry Ba2C3 containing linear C3 groups and is predicted to be stable in the pressure range 3-32 GPa. From our electron-phonon coupling calculations, the newly found BaC compound is a phonon-mediated superconductor and has a critical superconductivity temperature Tc of 4.32 K at 5 GPa. This compound is dynamically stable at 0 GPa and therefore may be quenchable under normal conditions.

Conversion of multilayer graphene into continuous ultrathin sp3-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp3-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between sp3 orbitals and metal surface dz2 orbitals. The induced electronic gap states and spin moments in the carbon layers are confined in a region within 0.5 nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms at the outer surface and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp3 carbon films on a metal surface that is diamagnetic, the strong covalent metal-sp3 carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our computational results suggest a route to experimental realization of large-area ultrathin sp3-bonded carbon films on metal surfaces.

Phonon-mediated superconductivity in silicene predicted by first-principles density functional calculations

We predict that electron-doped silicene is a good two-dimensional electron-phonon superconductor under biaxial tensile strain by first-principles calculations within the rigid-band approximation. Superconductivity transition temperature of electron-doped silicene can be increased up to above 10 K by 5% tensile strain. Band structures, phonon dispersive relations, and Eliashberg functions are calculated for detailed analysis. The strong interaction between acoustic phonon modes normal to the silicene plane and the increasing electronic states around the Fermi level induced by tensile strain is mainly responsible for the enhanced critical temperature.

Superconductivity in the Mn5Si3-type Zr5Sb3system

We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-structure type. Transport, magnetic, and calorimetric measurements and band structure calculations show it to be a phonon-mediated BCS superconductor, with a relatively large density of states at the Fermi level associated with the d-electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3. More superconductors with even higher transition temperatures are expected to be found in this family of compounds.

First-principles studies of the superconductivity and vibrational properties of transition-metal nitrides TMN (TM = Ti, V, and Cr)

The present paper reports the structural, electronic, phonon and thermodynamical properties of some transition-metal nitrides (TMN: TiN, VN and CrN) by means of first-principles calculations. The computed equilibrium lattice constant and bulk modulus agree well with the available experimental and theoretical data. The electronic band structure and density of states calculations show metallic nature. The phonon frequencies are positive throughout the Brillouin zone for these compounds in rocksalt structure indicating dynamical stability. The calculated electron–phonon coupling constant λ and superconducting transition temperature agree reasonably well with the available experimental data. These compounds behave as a conventional phonon-mediated superconductor. Within the GGA and quasi-harmonic approximation, thermodynamical properties are also investigated.

Electron–phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission

Based on the results from femtosecond time-resolved photoemission, we compare three different methods for the determination of the electron–phonon coupling constant λ in Eu- and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron–phonon-mediated superconductivity unlikely in these compounds.

Stability and superconductivity of Ca-B phases at ambient and high pressure

In the search for MgB${}_{2}$-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (T${}_{c}$) of 5.5 K. We have extended our previous study of CaB${}_{6}$ [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB${}_{6+x}$, finding that extra boron further stabilizes the proposed B${}_{24}$ units. Here an explanation is given for the transformation of cP7-CaB${}_{6}$ into the more complex oS56 and tI56 polymorphs at high pressure. The stability of the known metallic tP20 phase of CaB${}_{4}$ at ambient pressure is explained from a crystal structure and chemical bonding point of view. The tP20 structure is shown to destabilize at 19 GPa relative to a semiconducting MgB${}_{4}$-like structure due to chemical pressure from the metal ion. The hypothetical AlB${}_{2}$-type structure of CaB${}_{2}$, previously shown to have favorable superconducting features, is demonstrated here to be unstable at all pressures; two new metallic CaB${}_{2}$ polymorphs with unusual boron networks stabilize at elevated pressures above 8 GPa but are found to have very low critical temperatures (T${}_{c}$ $\ensuremath{\sim}$ 1 K). The stability of all structures has been rationalized through comparison with alkaline-earth analogs, emphasizing the importance of the size of the metal ion for the stability of borides. Our study illustrates the inverse correlation between the thermodynamic stability and superconducting properties and the necessity to carefully examine both in the design of new synthesizable superconducting materials.

Pressure-Induced Stabilization and Insulator-Superconductor Transition of BH

Diborane (B(2)H(6)), a high energy density material, was believed to be stable in a wide P, T interval. A systematic investigation of the B-H system using the ab initio variable-composition evolutionary simulations shows that boron monohydride (BH) is thermodynamically stable and can coexist with solid B, H(2), and B(2)H(6) in a wide pressure range above 50 GPa. B(2)H(6) becomes unstable and decomposes into the Ibam phase of BH and H(2) (C2/c) at 153 GPa. The semiconducting layered Ibam structure of BH at 168 GPa transforms into a metallic phase with space group P6/mmm and a 3D topology with strong B-B and B-H covalent bonds. The Ibam-P6/mmm transformation pathway suggests the possibility of obtaining the metastable Pbcm phase on cold decompression of the P6/mmm phase. The electron-phonon coupling calculations indicate that P6/mmm-BH is a phonon-mediated superconductor with a critical temperature of superconductivity (T(c)) of 14.1-21.4 K at 175 GPa.

Superconductivity in β-Tin Germanium

The superconducting properties of the β-tin germanium under pressure have been investigated by the first-principles calculations based on density functional perturbation theory. The electron–phonon coupling strength as a function of pressure was calculated, and it deceases with the application of pressure. Using the calculated electron–phonon coupling strength, the superconducting transition temperature T c has been estimated. Assuming a nominal Coulomb repulsion parameter of 0.1, the calculated T c is in excellent agreement with experimental data. The T c correlates with electron–phonon coupling strength, indicating phonon-mediated superconductivity in β-tin Ge phase. Our findings have great implications to other Group IVa elements.

Phonon-mediated superconductivity in quasi-1D Sc3CoC4

The mechanism for the superconductivity observed in quasi-1D Sc3CoC4 has been investigated with density functional theory. No evidence supporting a charge density wave distortion in the superconducting state was found. Instead, the conventional phonon-mediated theory was shown to be valid. Solutions to Eliashberg equations for partitioned vibrational modes permit the characterization of important electron-phonon processes. The superconducting state is the result of interactions between the electronic structure with the lattice vibrations and the dicarbo-C2 librations.

Intrinsic superconductivity in ABA-stacked trilayer graphene

We study the phonon-mediated superconductivity in light doped ABA-stacked trilayer graphene system by means of two theoretical models. We find superconducting transition temperature TC can be greatly enlarged by tuning the Fermi energy away from neutral point. Utilizing realistic parameters, we find Tc is approximately 1 K even under weak doping condition EF = 0.1 eV. Specifically, we give out the analytical expression for superconductivity gap △ and superconducting transition temperature Tc for negative-U Hubbard model. Further, we consider the thermal fluctuation and calculate the Berezinskii-Kosterlitz-Thouless critical temperature TBKT. Besides, we consider a two-band BCS model in comparision with the negative-U Hubbard model. The results for both models are qualitatively consistent. Our study provides a promising possibility for realizing intrinsic superconductivity in multilayer graphene systems.

High-temperature superconductivity in layered nitridesβ-LixMNCl (M=Ti, Zr, Hf): Insights from density functional theory for superconductors

We present an ab initio analysis with density functional theory for superconductors (SCDFT) to understand the superconducting mechanism of doped layered nitrides \beta-Li$_x$MNCl (M=Ti, Zr, and Hf). The current version of SCDFT is based on the Migdal-Eliashberg theory and has been shown to reproduce accurately experimental superconducting-transition temperatures Tc of a wide range of phonon-mediated superconductors. In the present case, however, our calculated Tc$\leq$4.3 K (M=Zr) and $\leq$10.5 K (M=Hf) are found to be less than a half of the experimental Tc. In addition, Tc obtained in the present calculation increases with the doping concentration x, opposite to that observed in the experiment. Our results indicate that we need to consider some elements missing in the present SCDFT based on the Migdal-Eliashberg theory.

Coexistence of Even- and Odd-Frequency Superconductivities Under Broken Time-Reversal Symmetry

A novel superconducting state under the broken time-reversal symmetry is studied in conventional phonon-mediated superconductors. By solving the Eliashberg equation self-consistently with the mass renormalization effect, it is found that the even- and odd-frequency components of the order parameter coexist in the bulk system as a consequence of the broken time-reversal symmetry. This finding would direct more attention to the odd-frequency pairing that affects physical quantities, especially in strong coupling superconductors.