BCS-type Superconductor Research Articles

Enhancement of critical current density and critical magnetic field of superconducting medium entropy alloy Nb2/5Hf1/5Zr1/5Ti1/5

We investigated the superconducting properties of medium entropy alloys (MEA) Nb2/5Hf1/5Zr1/5Ti1/5, synthesized by arc melting (AM) and powder metallurgical spark plasma sintering (SPS) methods. The samples exhibit superconducting transitions at Tc=7.99 K (AM) and 6.63 K (SPS) from the electrical resistivity measurements. The upper critical field at zero-temperature limit μoHc2(0) gives a relatively high value of 14.63 T (AM) and 11.68 T (SPS). The isothermal magnetic hysteresis of the SPS-MEA shows significantly enhanced vortex pinning, resulting in the enhancement of critical current density (Jc= 39,000 A/cm2) compared with the AM-MEA (Jc= 5,500 A/cm2) at 2 K (about seven times enhancement). The SPS-MEA shows a significant suppression of vortex avalanches compared with the previously reported SPS-HEA Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 superconductor. The SPS-MEA showed the secondary fishtail effect at a high magnetic field. There was a sizable specific heat jump ΔC(Tc)/γTc (3.3 for AM and 3.9 for SPS), significantly higher than the value of BCS prediction (1.43). Superconducting properties of MEA suggest the intermediate coupling of the electron-phonon coupling constant λ∼1 (λel−ph≃0.89 for AM and λel−ph≃0.78 for SPS) and the intermediate to strong coupling regime. The Kadowaki-Woods plot of the compounds is located on the line of the heavy fermion system. The Uemura plot indicates that the compounds are close to the conventional BCS superconductor. The contradictory property of strongly correlated and conventional BCS-type metallic superconductivity may come from the dirty superconducting regime with high Fermi velocity. Enhancing critical current density with significantly suppressed vortex avalanches on the SPS-MEA Nb2/5Hf1/5Zr1/5Ti1/5 compound suggests possible practical applications on the bulk superconductivity.

The validity of the BCS-like theory of Fermi-liquid superconductivity in overdoped cuprates

In this paper, we study the key and challenging issues arising in the physics of doped cuprate superconductors, which depending on the doping level (or the Fermi energy [Formula: see text]) and the characteristic energy [Formula: see text] of the attractive interaction between the charge carriers (holes or electrons) might be either in the limit of fermionic (weakly-bound) Cooper pairs just like in ordinary metals or in the limit of bosonic (tightly-bound) Cooper pairs. We argue that the underlying mechanism of superconductivity in doped cuprates depends on the fermionic or bosonic nature of Cooper pairs. We examine the validity criterion of the Bardeen–Cooper–Schrieffer (BCS)-like theory of Fermi-liquid superconductivity in these materials and determine the doping level at which this criterion is well satisfied. We find that the specific overdoped cuprates (with [Formula: see text] or [Formula: see text][Formula: see text]eV) can become the BCS-type superconductors. Using a pertinent BCS-like pairing theory, we calculate the superconducting order parameter (defined as a BCS-like gap [Formula: see text]) and the critical temperature [Formula: see text] of the superconducting transition and the doping dependences of [Formula: see text] and [Formula: see text] in various overdoped cuprates. Our results for [Formula: see text] and [Formula: see text] in specific overdoped cuprates are in fair agreement with the experimental findings reported for these systems.

Prediction of superconductivity in Li, K, Ca, and Sr-intercalated blue phosphorene bilayer using first-principle calculations

Blue phosphorene is an interesting two-dimensional (2D) material, which has attracted the attention of researchers, due to its affluent physical and chemical properties. In recent years, it was discovered that the intercalation of alkali metals and alkaline earth metals in 2D materials may lead to conventional Bardeen–Cooper–Schrieffer (BCS) superconductivity. In this work, the electronic structure, phonon dispersion, Eliashberg spectral function, electron–phonon coupling (EPC), and the critical temperature of blue phosphorene bilayer intercalated by alkali metals (Li, and K) and alkaline earth metals (Ca, and Sr) for both AB and AC stacking orders are studied using the density functional theory and the density functional perturbation theory, within the generalized gradient approximation with van der Waals correction. The present work shows that the blue phosphorene bilayer is dynamically stable in AB stacking for Li and AC stacking for K, Ca, and Sr, and after intercalation, it transforms from a semiconductor to a metal owing to charge transfer between intercalated atoms and phosphorene. Furthermore, the EPC constant and the critical temperature are higher than those of 2D BCS-type superconductors. They are about 3 and 24.61 K respectively for K-intercalated blue phosphorene bilayer. Thus, our results suggest that blue phosphorene is a good candidate for a superconductor.

Role of the Berry curvature on BCS-type superconductivity in two-dimensional materials

We theoretically investigate how the Berry curvature, which arises in multiband structures when the electrons can be described by an effective single-band Hamiltonian, affects the superconducting properties of two-dimensional electronic systems. Generically, the Berry curvature is coupled to electric fields beyond those created by the periodic crystal potential. A potential source of such electric fields, which vary slowly on the lattice scale, is the mutual interaction between the electrons. We show that the Berry curvature provides additional terms in the Hamiltonian describing interacting electrons within a single band. When these terms are taken into account in the framework of the usual BCS weak-coupling treatment of a generic attractive interaction that allows for the formation of Cooper pairs, the coupling constant is modified. In pure singlet and triplet superconductors, we find that the Berry curvature generally lowers the coupling constant and thus the superconducting gap and the critical temperature as a function of doping. From an experimental point of view, a measured deviation from the expected BCS critical temperature upon doping, e.g., in doped two-dimensional transition-metal dichalcogenides, may unveil the strength of the Berry curvature.

Negative Chemical Pressure Effect on the Superconductivity and Charge Density Wave of Cu0.5Ir1–xZrxTe2

This study demonstrates the design and synthesis of Cu0.5Ir1-xZrxTe2 system by partial substitution of Ir with Zr acting as a negative chemical pressure. With the doping of Zr, the cell parameters significantly expand, signifying an effective negative chemical pressure. The experimental results found evidence that the charge density wave (CDW)-like order is immediately quenched by subtle Zr substitution for Ir and a classical dome-shape Tc(x) that peaked at 2.80 K can be observed. The optimal Cu0.5Ir0.95Zr0.05Te2 compound is a BCS-type superconductor and exhibits type-II SC. However, high Zr concentration can provoke disorder, inducing the reappearance of CDW order. The present study shows that the Cu0.5Ir1-xZrxTe2 system may provide a new platform for further understanding of multiple electronic orders in transition metal dichalcogenides.

Elucidating the underlying mechanism of relatively high T c value of the orthorhombic MoRuP: a first-principles study

ABSTRACT In this paper, the structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties of orthorhombic MoRuP have been analysed by using the generalised gradient approximation of the density functional theory and the plane-wave pseudopotential method. An examination of elastic constants and elastic moduli indicates that the orthorhombic MoRuP is mechanical stable and ductile. A critical assessment of electronic structure and electronic density of states implies that the electronic states in the vicinity of the Fermi level are mostly composed of the 4d states of the molybdenum atoms in MoRuP. A comparison of Eliashberg spectral function with electronic and phonon density of states reveals that the mechanism behind the superconductivity for the orthorhombic MoRuP is the coupling of Mo 4d electrons with Mo-related lattice vibrations. The integration of Eliashberg spectral function gives the electron-phonon coupling parameter of MoRuP to be 0.98, implying that the orthorhombic MoRuP belongs to the conventional BCS-type superconductors with strong coupling. Finally, the value of superconducting transition temperature for MoRuP amounts to 16.5 K, harmonising with the experimental value of 15.5 K.

Non-BCS-type superconductivity and critical thickness of SrTiO3/LaAlO3/SrTiO3 trilayer interface system

As a quest for two-dimensional conducting interface with exotic functionalities for future electronic devices, the perovskite heterointerface of LaAlO3/SrTiO3 (LAO/STO) has been intensively studied. For the LAO/STO heterostructure, the critical thickness of the LAO layer for metallic conduction is 4 unit cells (uc) according to the polar catastrophe scenario. However, we find that metallic conduction can also be induced at the LAO/STO interface with only 1 uc of LAO, as long as it is capped by 3 or more uc of STO. Consistent results are obtained from density functional theory calculations. For this STO/LAO/STO trilayer, we also confirmed a peculiar non-BCS-type superconductivity with a suppressed superconducting gap, which may imply a superconducting coupling distinct from previously reported BCS-type superconductivity in LAO/STO heterointerface. These observations suggest that the STO/LAO/STO trilayer can be another testing board to explore carrier conduction in two-dimensional electron systems for electronic device applications utilizing exotic functionalities.

Fully gapped superconductivity in centrosymmetric and noncentrosymmetric Re-B compounds probed with μSR

We present a comprehensive study on superconducting properties of Re$_7$B$_3$ and Re$_3$B through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation ($\mu$SR) experiments on polycrystalline samples. Re$_7$B$_3$ (T$_C$ = 3.2~K) is a non-centrosymmetric type-II ($\kappa$ $\approx$ 9.27) superconductor in the weak coupling ($\lambda_{e-ph}$ = 0.54) regime. On the other hand, Re$_3$B (T$_C$ = 5.19~K) is a centrosymmetric type-II ($\kappa$ $\approx$ 34.55) superconductor in the moderate coupling ($\lambda_{e-ph}$ = 0.64) regime. Our transverse-field $\mu$SR measurements show evidence for isotropically gapped BCS type superconductivity with normalized gap ($\Delta_0/k_BT_C$) values of 1.69 (Re$_7$B$_3$) and 1.75 (Re$_3$B).

Collective modes and terahertz near-field response of superconductors

We theoretically study the low energy electromagnetic response of BCS type superconductors focusing on propagating collective modes that are observable with THz near field optics. The interesting frequency and momentum range is $\omega < 2\Delta$ and $q < 1/\xi$ where $\Delta$ is the gap and $\xi$ is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes and Carlson-Goldman modes using THz near field technique, although none of these modes couple linearly to far field radiation. Coupling of THz near field radiation to the amplitude mode requires particle-hole symmetry breaking while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near field reflection coefficient as a weak feature in the sub-THz frequency range. In a system of two superconducting layers with nanometer scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well defined resonance peaks in the near-field THz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two dimensional superconductors, realized for example in samples of high T$_c$ cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the THz near field radiation.

Specific criteria for BCS-type cuprate superconductivity and peculiar isotope effects on the critical superconducting transition temperature

So far, many researchers have been misled to believe that the Bardeen–Cooper–Schrieffer (BCS)-like (s- or d-wave) pairing theory is adequate for explaining high- $$T_\mathrm {c}$$ superconductivity in doped cuprates from underdoped to overdoped regime. We show that the doped cuprates, depending on the Fermi energy ( $$\varepsilon _\mathrm {F}$$ ) and the energy ( $$\varepsilon _\mathrm {A}$$ ) of the effective attraction between pairing carriers, might be either unconventional (non-BCS-type) superconductors (at intermediate doping) or BCS-type superconductors (at higher doping). We argue that specific criteria for BCS-type superconductivity formulated in terms of two ratios $$\varepsilon _\mathrm {A}/\varepsilon _\mathrm {F}$$ and $$\Delta /\varepsilon _\mathrm {F}$$ (where $$\Delta $$ is the BCS-like gap) must be met in these systems. We demonstrate that these criteria are satisfied only in overdoped cuprates but not in underdoped and optimally doped cuprates, where the origin of high- $$T_\mathrm {c}$$ superconductivity is quite different from the BCS-type (s- or d-wave) superconductivity. The BCS-like pairing theory is then used to calculate the critical superconducting transition temperature ( $$T_\mathrm {c}$$ ) and the peculiar oxygen and copper isotope effects on $$T_\mathrm {c}$$ in overdoped cuprates.

Superconductivity of Y5Rh6Sn18; Coexistence of the high temperature thermal lattice relaxation process and superconductivity

The heat capacity, magnetic, and electrical transport properties of skutterudite-related Y5Rh6Sn18 compound indicate intriguing coexistence of the high temperature thermal lattice relaxation process at temperatures higher than 350 K and superconductivity below the superconducting transition temperature Tc=3.08 K. Y5Rh6Sn18, having a cage-like structure crystallises in tetragonal structure with the space group I41/acd. The gap structure of this compound is found not to be characteristic of conventional BCS type superconductors. Anomalous behaviour observed in the electronic specific heat C(T)/T and the upper critical field Hc2(T) data may suggest the presence of two superconducting gaps in this compound. The ab initio band structure calculations showed a significant contribution of s and p Sn states as well as Rh and Y d-electron states to the total density of states near the Fermi level, which supports the multi-band superconductivity of this compound. Several electronic quantities and parameters characterising the superconducting and normal state of Y5Rh6Sn18 are calculated within the Ginzburg-Landau-Abrikosov-Gorkov theory. The data reveals dirty limit superconductivity in Y5Rh6Sn18 superconductor of type-II.

Enhanced upper critical field in Co-doped Ba122 superconductors by lattice defect tuning

Nanoscale defects in superconductors play a dominant role in enhancing superconducting properties through electron scattering, modulation of coherence length, and correlation with quantized magnetic flux. For iron-based superconductors (IBSCs) that are expected to be employed in high-field magnetic applications, a fundamental question is whether such defects develop an upper critical field (Hc2) similar to that of conventional BCS-type superconductors. Herein, we report the first demonstration of a significantly improved Hc2 in a 122-phase IBSC by introducing defects through high-energy milling. Co-doped Ba122 polycrystalline bulk samples [Ba(Fe, Co)2As2] were prepared by sintering powder which was partially mechanically alloyed through high-energy milling. A remarkable increase in the full-width at half maximum of X-ray powder diffraction peaks, anomalous shrinkage in the a-axis, and elongation in the c-axis were observed. When lattice defects are introduced into the grains, the semiconductor behavior of the electric resistivity at a low temperature (T &amp;lt; 100 K), a slight decrease in transition temperature (Tc), an upturn of Hc2(T) near Tc, and a large increase in the Hc2(T) slope were observed. The slope of Hc2(T) increased approximately by 50%, i.e., from 4 to 6 T/K, and exceeded that of single crystals and thin films. Defect engineering through high-energy milling is expected to facilitate new methods for the designing and tuning of Hc2 in 122-phase IBSCs.

Superconducting behavior of a new metal iridate compound, SrIr2, under pressure

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, Tc, as high as 6.6 K at ambient pressure, in order to complete the Tc—pressure (p ) phase diagram. Very recently, this sample’s superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample’s superconductivity in a wide pressure range. The Tc value decreased with an increase in pressure, but the onset superconducting transition temperature, , increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R − T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

Special role of the first Matsubara frequency for superconductivity near a quantum critical point: Nonlinear gap equation below Tc and spectral properties in real frequencies

Near a quantum-critical point in a metal a strong fermion-fermion interaction, mediated by a soft boson, destroys fermionic coherence and also gives rise to superconductivity. In a class of large $N$ models, one would naively expect an incoherent (non-Fermi liquid) normal state behavior to persist down to $T=0$. However, this is not the case for quantum-critical systems described by Eliashberg theory, where non-Fermi liquid part of the self-energy $\Sigma (\omega_m)$ is large for a generic Matsubara frequency $\omega_m = \pi T(2m+1)$, but vanishes at $\omega_m = \pm \pi T$, while the pairing interaction between fermions with these two frequencies remains strong. This peculiarity gives rise to a non-zero $T_c$ even at large $N$[Y. Wang et al PRL 117, 157001 (2016)]. We consider the system behavior below $T_c$ and contrast the two cases when $\omega_m = \pm \pi T$ are either special or not. We obtain the solution of the non-linear gap equations in real frequency axis and obtain the spectral function $A(\omega)$ and the density of states $N(\omega)$. In a conventional BCS-type superconductor $A(\omega)$ and $N(\omega)$ are peaked at the gap value $\Delta (T)$, and the peak position shifts to a smaller $\omega$ as temperature increases, i.e. the gap "closes in". However when superconductivity is induced by fermions with $\omega_m = \pm \pi T$, the peak in $N(\omega)$ remains at a finite frequency even at $T =T_c-0$, the gap just "fills in". The spectral function $A(\omega)$ either shows almost the same "gap filling" behavior, or its peak position shifts to zero frequency already at a finite $\Delta$ ("emergent Fermi arc" behavior), depending on the strength of the thermal contribution. We compare our results with the data for the cuprates and argue that "gap filling" behavior holds in the antinodal region, while the "emergent Fermi arc" behavior holds in the nodal region.

Microscopic mechanism of room-temperature superconductivity in compressed LaH10

Room-temperature superconductivity has been one of the most challenging subjects in modern physics. Recent experiments reported that lanthanum hydride LaH$_{10{\pm}x}$ ($x$$<$1) raises a superconducting transition temperature $T_{\rm c}$ up to ${\sim}$260 (or 215) K at high pressures around 190 (150) GPa. Here, based on first-principles calculations, we reveal the existence of topological Dirac-nodal-line (DNL) states in compressed LaH$_{10}$. Remarkably, the DNLs protected by the combined inversion and time-reversal symmetry and the rotation symmetry create a van Hove singularity (vHs) near the Fermi energy, giving rise to large electronic density of states. Contrasting with other La hydrides containing cationic La and anionic H atoms, LaH$_{10}$ shows a peculiar characteristic of electrical charges with anionic La and both cationic and anionic H species, caused by a strong hybridization of the La $f$ and H $s$ orbitals. We find that a large number of electronic states at the vHs are strongly coupled to the H-derived high-frequency phonon modes that are induced via the unusual, intricate bonding network of LaH$_{10}$, thereby yielding a high $T_{\rm c}$. Our findings not only elucidate the microscopic origin of the observed high-$T_{\rm c}$ BCS-type superconductivity in LaH$_{10}$, but also pave the route for achieving room-temperature topological superconductors in compressed hydrogen-rich compounds.

Emergence of superconductivity in a Dirac nodal-line Cu2Si monolayer: ab initio calculations

We demonstrate that the recent experimentally synthesized Cu2Si monolayer is a BCS-type two-dimensional superconductor with Tc being ∼ 4.1 K.

Effect of pressure on the physical properties of the superconductor NiBi3

We present an experimental study of the superconducting properties of NiBi3 as a function of pressure by means of resistivity and magnetization measurements and combine our results with density functional theory calculations of the band structure under pressure. We find a moderate suppression of the critical temperature from K to K by pressures up to 2 GPa. By taking into account the change of the band structure as a function of pressure, we argue that the decrease in is consistent with conventional, electron–phonon-mediated BCS-type superconductivity.

Superconducting Ti15Zr15Nb35Ta35 High-Entropy Alloy With Intermediate Electron-Phonon Coupling

The body-centered cubic (BCC) Ti15Zr15Nb35Ta35 high-entropy alloy showed superconducting behavior at around 8 K. The electronic specific heat coefficient and the lattice specific heat coefficient β were determined to be γ=9.3±0.1 mJ/mol K2 and β=0.28±0.01 mJ/mol K4, respectively. It was found that the electronic specific heat Ces does follow the exponential behavior of the Bardeen-Cooper-Schrieffer (BCS) theory. Nevertheless, the specific heat jump (ΔC/γTc ) at the superconducting transition temperature which was determined to be 1.71 deviates appreciably from that for a weak electron-phonon coupling BCS superconductor. Within the framework of the strong-coupled theory, our analysis suggests that theTi15Zr15Nb35Ta35 HEA is an intermediate electron-phonon coupled BCS-type superconductor.

Theoretical investigation of superconductivity in the non-centrosymmetric SrPtGe3 and CaPtSi3 compounds

First-principles calculations are made to investigate structural, elastic, electronic, vibrational and electron–phonon interaction properties of non-centrosymmetric superconductors SrPtGe3 and CaPtSi3. Electronic structure calculation results show that the density of states at the Fermi level for both compounds mainly comes from the p electrons of Ge (or Si), which considerably hybridise with Pt d states. The calculated Eliashberg spectral function with and without spin–orbit interaction reveals that while all phonon branches are involved in the process of scattering of electrons near the Fermi level, the average electron–phonon coupling parameter values of 0.47 for SrPtGe3 and 0.49 for CaPtSi3 indicate that these compounds are phonon-mediated BCS-type superconductors with a weak strength. By using a reasonable value of = 0.11 for the effective Coulomb repulsion parameter, the superconducting transition temperatures for SrPtGe3 and CaPtSi3 are found to be 1.3 K and 2.4 K, respectively, which compare very well with the corresponding experimental values of 1.0 K and 2.3 K.

The σ-phase superconductors Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9

We show that the previously unreported ternary σ phases Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are both superconductors with Tc values of approximately 1.9 K. The superconducting transitions were characterized through temperature-dependent magnetic susceptibility, electrical resistance, and specific heat measurements. The Sommerfeld constants, γ, for Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are 89(1) mJ mol-f.u.−1K−2 and 86(1) mJ mol-f.u.−1K−2 (about 3.0 and 2.9 mJ mol-atom−1 K−2), respectively. The normalized specific heat jumps at Tc, ΔC/γTc, are approximately 1.39 for Nb20.4Rh5.7Ge3.9 and 1.30 for Nb20.4Rh5.7Si3.9, consistent with both materials being weakly coupled BCS type superconductors.