Presence Of Superconductivity Research Articles

Variational Monte Carlo study of stripes as a function of doping in the t−t′ Hubbard model

We perform variational Monte Carlo simulations of the single-band Hubbard model on the square lattice with both nearest (t) and next-nearest (t′) neighbor hoppings. Our work investigates the consequences of increasing hole doping on the instauration of stripes and the behavior of the superconducting order parameter, with a discussion on how the two phenomena affect each other. We consider two different values of the next-nearest neighbor hopping parameter, that are appropriate for describing cuprate superconductors. We observe that stripes are the optimal state in a wide doping range; the stripe wavelength reduces at increasing doping, until stripes melt into a uniform state for large values of doping. Superconducting pair–pair correlations, indicating the presence of superconductivity, are always suppressed in the presence of stripes. Our results suggest that the phase diagram for the single-band Hubbard model is dominated by stripes, with superconductivity being possible only in a narrow doping range between striped states and a nonsuperconducting metal.

Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2.

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Superconducting properties of new hexagonal and noncentrosymmetric cubic high entropy alloys

Superconducting high-entropy alloys (HEAs) are a newly burgeoning field of unconventional superconductors and raise intriguing questions about the presence of superconductivity in highly disordered systems, which lack regular phonon modes. In our study, we have synthesized and investigated the superconducting characteristics of two new transition elements based HEAs Re0.35Os0.35Mo0.10W0.10Zr0.10 crystallizing in noncentrosymmetric α-Mn structure, and Ru0.35Os0.35Mo0.10W0.10Zr0.10 crystallizing in hexagonal closed-packed structure (hcp). Due to its high hardness, transition-metal-based hexagonal hcp HEA is rare and highly desirable for practical applications. Bulk magnetization, resistivity, and specific heat measurements confirmed bulk type-II superconductivity in both alloys. Specific heat analysis up to the measured low-temperature range suffices for a Bardeen-Cooper-Schrieffer explanation.

Single crystal synthesis, structure, and magnetism of Pb10−xCux(PO4)6O

The recent claim of superconductivity above room temperature in Pb10−xCux(PO4)6O with 0.9 < x < 1 (referred to as LK-99) has sparked considerable interest. To minimize the influence of structural defects and impurity phases on the physical properties, we have synthesized phase-pure single crystals with a copper doping level of x ∼ 1. We find that the crystals are highly insulating and optically transparent. X-ray analysis reveals an uneven distribution of the substituted Cu throughout the sample. Temperature (T) dependent magnetic susceptibility measurements for 2 ≤ T ≤ 800 K reveal the diamagnetic response characteristic of a non-magnetic insulator, as well as a small ferromagnetic component, possibly originating from frustrated exchange interactions in Cu-rich clusters in the Pb10−xCux(PO4)6O structure. No anomalies indicative of phase transitions are observed. We, therefore, rule out the presence of superconductivity in Pb9Cu(PO4)6O crystals and provide some considerations on the origin of anomalies previously reported in experiments on polycrystalline specimens.

Prediction of phonon-mediated superconductivity and topological surface states in NbRu[formula omitted

Based on the first-principles calculations, we obtain the superconducting transition temperature (Tc) of 21.17 K for NbRu3C. For the superconductivity of NbRu3C, the strong electron–phonon coupling mainly comes from the coupling between Ru 4d orbitals and low-energy phonon modes which include triply degenerate T1u and doubly degenerate E. Moreover, NbRu3C is found to have a topological insulating phase, confirmed by the calculated nontrivial surface states and spin textures. The presence of superconductivity and topological surface states in NbRu3C suggests that NbRu3C may be a candidate for the topological superconductor.

Corner states, hinge states, and Majorana modes in SnTe nanowires

SnTe materials are one of the most flexible material platforms for exploring the interplay of topology and different types of symmetry breaking. We study symmetry-protected topological states in SnTe nanowires in the presence of various combinations of Zeeman field, s-wave superconductivity and inversion-symmetry-breaking field. We uncover the origin of robust corner states and hinge states in the normal state. In the presence of superconductivity, we find inversion-symmetry-protected gapless bulk Majorana modes, which give rise to quantized thermal conductance in ballistic wires. By introducing an inversion-symmetry-breaking field, the bulk Majorana modes become gapped and topologically protected localized Majorana zero modes appear at the ends of the wire.

Superconductivity assisted change of the perpendicular magnetic anisotropy in V/MgO/Fe junctions

Controlling the perpendicular magnetic anisotropy (PMA) in thin films has received considerable attention in recent years due to its technological importance. PMA based devices usually involve heavy-metal (oxide)/ferromagnetic-metal bilayers, where, thanks to interfacial spin-orbit coupling (SOC), the in-plane (IP) stability of the magnetisation is broken. Here we show that in V/MgO/Fe(001) epitaxial junctions with competing in-plane and out-of-plane (OOP) magnetic anisotropies, the SOC mediated interaction between a ferromagnet (FM) and a superconductor (SC) enhances the effective PMA below the superconducting transition. This produces a partial magnetisation reorientation without any applied field for all but the largest junctions, where the IP anisotropy is more robust; for the smallest junctions there is a reduction of the field required to induce a complete OOP transition (H_text {OOP}) due to the stronger competition between the IP and OOP anisotropies. Our results suggest that the degree of effective PMA could be controlled by the junction lateral size in the presence of superconductivity and an applied electric field. We also discuss how the H_text {OOP} field could be affected by the interaction between magnetic stray fields and superconducting vortices. Our experimental findings, supported by numerical modelling of the ferromagnet-superconductor interaction, open pathways to active control of magnetic anisotropy in the emerging dissipation-free superconducting spin electronics.

Revisiting the influence of Fe excess in the synthesis of BaFe2S3

BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic and transport properties. Here, we present a systematic study of compositional, structural and physical properties of BaFe$_{2+\delta}$S$_3$ as a function of the nominal Fe excess $\delta$. As $\delta$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. The previously reported maximum of $T_N$ at $\delta$=0.1 was not confirmed. Samples with nominal $\delta$=0 present the lowest $T_N$ and the resistivity anomaly at the highest temperature $T^*$ while, for $\delta \geq 0.05$, both quantities and the transport gap are seemingly $\delta$-independent. Finally, we show that crystals free of ferromagnetic spurious phases can be obtained by remelting samples with nominal $\delta$=0.05 in a Bridgman process.

Laser-Assisted Floating Zone Growth of BaFe2S3 Large-Sized Ferromagnetic-Impurity-Free Single Crystals

The quasi-one-dimensional antiferromagnetic insulator BaFe2S3 becomes superconducting under a hydrostatic pressure of ∼10 GPa. Single crystals of this compound are usually obtained by melting and further slow cooling of BaS or Ba, Fe, and S, and are small and needle-shaped (few mm long and 50–200 μm wide). A notable sample dependence on the antiferromagnetic transition temperature, transport behavior, and presence of superconductivity has been reported. In this work, we introduce a novel approach for the growth of high-quality single crystals of BaFe2S3 based on a laser-assisted floating zone method that yields large samples free of ferromagnetic impurities. We present the characterization of these crystals and the comparison with samples obtained using the procedure reported in the literature.

Absence of superconductivity in bulk Nd1\u2212xSrxNiO2

Superconductivity at 9–15 K was recently discovered in Nd0.8Sr0.2NiO2 films. Since the Ni1+ ionic state in NdNiO2 may have the same 3d9 outer-shell electronic orbital as in cuprate superconductors, it is interesting to know whether superconductivity has a similar mechanism in these two systems. Here we synthesize bulk samples of Nd1−xSrxNiO2 (x = 0, 0.2, 0.4) with inhomogeneous Sr distribution and Ni deficiency. Resistivity measurements show insulating behavior without the presence of superconductivity, different to the previously reported films. Although applying pressure up to about 50.2 GPa significantly suppresses the insulating behavior, superconductivity remains absent. The magnetization behavior exhibits a Curie–Weiss law with a paramagnetic moment of about 2 μB/f.u. Since the lattice constants derived from our diffraction data are very close to the previously reported superconducting Nd0.8Sr0.2NiO2 films, we suggest that superconductivity in that system may have arisen from interface or stress-related effects, or nickel deficiency in our bulk samples that might prevent the emergence of superconductivity.

Chemical and structural stability of superconducting In5Bi3 driven by spin–orbit coupling

Relativistic effects play a prominent role in many electronic material properties such as the Rashba and Dresselhaus spin splitting in inversion asymmetric crystals, or the bulk band gap in topological insulators. By contrast, macroscopic material properties are typically not connected to relativistic phenomena. As an exception to this rule, we show that the macroscopic chemical and structural properties of superconducting In5Bi3 are driven by relativistic physics. In the non-relativistic limit In5Bi3 decomposes into elemental indium and bismuth, but the inclusion of relativistic spin–orbit coupling chemically stabilizes the In5Bi3 stoichiometry. Similarly, the structural stability of tetragonal In5Bi3 is driven by the spin–orbit interaction, which eliminates a phonon instability present in the non-relativistic limit. Low-temperature resistivity and heat capacity measurements show that In5Bi3 is a strong coupling superconductor, with a superconducting critical temperature of 4.2 K and a superconducting critical field of 0.3 T. The unconventional interplay between relativity with chemistry and structure, together with the presence of superconductivity, make In5Bi3 a versatile material that provides, for example, a simple model for the study of strong coupling superconductivity in quasiperiodic crystals.

Spin caloritronics in a chiral double-strand-DNA-based hybrid junction

The chiral-induced spin selectivity effect is of fundamental importance in further understanding the thermospin transport in double-strand-DNA (dsDNA) molecules. Here we propose a hybrid dsDNA-based thermoelectric device made of a chiral dsDNA molecule coupled to normal-metal and superconducting leads for exploring the spin-dependent thermoelectricity. We give a comparison of the thermoelectric responses of the dsDNA system and the quantum dot system in the presence of superconductivity, and find that the former does not rely on external means to break the particle-hole symmetry due to the intrinsic properties of dsDNA molecules. Our findings show that spin-thermoelectric coefficients can go beyond charge thermoelectric ones, and even a pure spin thermopower can be generated by adjusting the external magnetic field and the temperature, which indicates that this chiral dsDNA hybrid junction can in practice act as a pure spin-current generator. Furthermore, the sizable spin and charge thermoelectric conversion efficiencies can be achieved by tuning system parameters. Our results open opportunities for fabricating spin caloritronic devices based on chiral molecules.

Modification of transfer-matrix method for electromagnetic waves in layered superconductor in presence of dc magnetic field

In the present paper, we modify the transfer-matrix method to study the dissipation-free transition of electromagnetic waves of terahertz range through a plate of layered superconductor embedded in the dielectric environment in the presence of external direct current (dc) magnetic field. In this work, we сonsider TM-polarized electromagnetic waves. The setup is arranged in such a way that the dielectric and superconducting layers in the plate are perpendicular to its interface, and the external magnetic field is directed along the plate and parallel to the layers. We consider the case of a weak external dc field at which magnetic vortices do not penetrate the plate. Due to the nonlinearity of the Josephson plasma formed in the layered superconductor, the dc magnetic field penetrates non-uniformly into the plate and affects the electromagnetic wave. Hence, the magnitude of the external dc magnetic field can be used as a variable parameter to tune various phenomena associated with the propagation of an electromagnetic waves in layered superconductors. In the presence of the external homogeneous dc magnetic field, linear electromagnetic waves in the layered superconductor turn out to be non-exponential. Therefore we cannot directly apply the transfer matrix method, in which the amplitudes of the corresponding exponents are compared. However, in the present paper, it is shown that for a sufficiently thick plate, the matrices describing the wave transfer through the plate can be introduced. The analytical expressions for these matrices are derived explicitly in terms of special Legendre functions. The obtained transfer-matrices can be used for the further study of the wave transfer through the layered superconductor in the presence of an external dc magnetic field.

Study of activation energy and AC-susceptibility for nano-ZnO doped MgB2 superconductor in presence of varying amplitude of applied field

AC-susceptibility dependence of temperature measurements with varying applied magnetic field is studied for nano-ZnO doped MgB2 superconductors at different doping levels along with pristine material. The home-made susceptometer is used to measure the AC-susceptibility at different varying field (H ac = 0.2, 0.8, 4.0, 20.0 and 40.0 A/cm). The AC-susceptibility of the material shows the magnetic properties of that material either attracting or repelling in nature. The purpose of choosing the MgB2 superconductor is that because it has simple crystal structure with profound properties such as two band gaps. Generally, the AC-susceptibility has two parts, real and imaginary. The peak height of the imaginary part of the AC-susceptibilities is enhancing by increasing the applied field and shifted to the lower temperature. The real part of AC-susceptibility as a function of temperature under the presence of varying applied fields illustrates the minutely drop of the transition temperature, T c. All the samples were cooled down from 300 to 4 K in presence of applied field. The relaxation time is calculated using by optimum frequency (f = 98 Hz). The activation energy as a function of current density is measured with different doped and pure MgB2 samples. In addition, this activation energy as a function of current density is also determined under the effect of the applied field. The collective pinning model was adopted in order to calculate the activation energy as a function of current density. The current density is found to be increased as the activation energy increases in both the cases.

Bound state, phase separation and superconductivity in presence of Rashba spin-orbit coupling

We have investigated the phase diagram for the t−J model at low electronic densities in presence of Rashba spin-orbit coupling (RSOC). We have rigorously derived a bound state criterion which arises out of a competition between the kinetic energy of the electrons and the exchange coupling between them. Further, we have obtained that the phase diagram consists of three phases, namely, a gas of electrons, a gas of bound pairs, and a fully phase separated state. Subsequently an extension of the pairing scenario is done at finite densities by solving a BCS gap equation. Finite superconducting correlations are observed for J values much lower than that required for the formation of a single bound pair, thereby indicating that pairing in a many particle environment requires weaker interaction strengths than that in the dilute case. We have further obtained that the RSOC increases the transition temperature for a p-wave pairing state, while it diminishes the same for an s-wave pairing correlations.

Effect of external magnetic field on the coexistence of SC and AFM in iron based superconductors

We have studied the interplay of antiferromagnetism and superconductivity in presence of an applied external magnetic field for the iron based superconductors. For the purpose we have proposed a model Hamiltonian and solved it self-consistently by using the Zubarev's technique of double time Green's function technique. The self-consistent gap equations are solved numerically and interpreted the gap values from the of density of states plots.

Surface superconductivity in presence of corners

We consider an extreme type-II superconducting wire with non-smooth cross section, i.e. with one or more corners at the boundary, in the framework of the Ginzburg–Landau theory. We prove the existence of an interval of values of the applied field, where superconductivity is spread uniformly along the boundary of the sample. More precisely, the energy is not affected to leading order by the presence of corners and the modulus of the Ginzburg–Landau minimizer is approximately constant along the transversal direction. The critical fields delimiting this surface superconductivity regime coincide with the ones in absence of boundary singularities.

The influence of magnetic steps on bulk superconductivity

We study the distribution of bulk superconductivity in presence of an applied magnetic field, supposed to be a step function, modeled by the Ginzburg-Landau theory. Our results are valid for the minimizers of the two-dimensional Ginzburg-Landau functional with a large Ginzburg-Landau parameter and with an applied magnetic field of intensity comparable with the Ginzburg-Landau parameter.

Effect of Sodium Substitution on Structural and Magnetic Properties of KFe2−y Se2

We have synthesized the superconducting K1−xNaxFe2−ySe2 (x = 0.07 and 0.25) crystal by using the flux technique. SEM images demonstrate that two samples consist of successive plate-like surfaces oriented into ab plane. EDS analysis yields the average compositions as (Na0.20K0.80)0.96Fe1.78Se2for x = 0.07 and (Na0.33K0.67)0.96Fe1.73Se2 for x = 0.25. XRD pattern gives the (00l) peaks. The main peaks are indexed to the I4/m space group corresponding to reflected intensities from (002), (006), (008), and (0010) planes related to the peaks of the phase K2Fe4Se5. The lattice parameters are calculated as a = 8.701(4) and c = 14.185(2) A for x = 0.07, and a = 8.689(3) and c = 14.054(4) A for x = 0.25. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperatures, Tc, obtained from magnetization measurements are estimated to be 31.0 and 28.4 K for x = 0.07 and 0.25, respectively. The critical current value, Jc(0), deduced from the M−H loops are approximately 2.8 ×104 A/cm 2 for x = 0.07 and 2.87 ×102 A/cm 2 for x = 0.25 at 5 K. The volume pinning force, Fp, of x = 0.07 sample is found larger than that of the x = 0.25 one.

XPS study of the electronic density of states in the superconducting Mo2B and Mo2BC compounds

The electronic structure of the Mo\(_2\)BC and Mo\(_2\)B compounds was investigated by X-ray photoelectron spectroscopy. The Mo 3d, C 1s, and B 1s core levels are identified. For the Mo\(_2\)BC, the core-level binding energies corresponding to Mo 3d\(_{5/2}\), B 1s, and C 1s are localized at 227.90, 187.94, and 282.95 eV, respectively, whereas for the Mo\(_2\)B, the Mo 3d\(_{5/2}\), and B 1s are localized at 228.09 and 188.06 eV, respectively. Core-level binding energies shifts are observed in both compounds using the charge-potential model. The electronic density of states was calculated for Mo\(_2\)B and Mo\(_2\)BC using GGA approximation. Our results show that the electronic density of states at the Fermi level in the Mo\(_2\)B is higher than that in the Mo\(_2\)BC. The dominance of the Mo 4d states down to 8 eV below the Fermi level is found. The calculated total DOS was consistent with the XPS valence band spectra. Finally, within the BCS theory framework, the presence of superconductivity in both compounds can not be explained only as a function of the electronic density of states at the Fermi level. The electron-phonon coupling constant (\(\lambda \)) was calculated using the McMillan equation; the obtained values were 0.75 for Mo\(_{2}\)BC and 0.70 for Mo\(_{2}\)B. These values indicate that both compounds are intermediate coupled superconductors.