Superconducting Phase Research Articles

Pressure-induced suppression of charge density wave and emergence of superconductivity in 1T−VSe2

We report pressure evolution of charge density wave (CDW) order and emergence of superconductivity (SC) in 1T-VSe2 single crystal by studying resistance and magnetoresistance behavior under high pressure. With increasing quasi-hydrostatic pressure the CDW order enhances with increase ofthe ordering temperature up to 240K at 12 GPa. Upon further increase of pressure, the resistance anomaly due to CDW order gets suppressed drastically and superconductivity emerges at ~15 GPa, with the onset critical temperature (Tc) ~ 4K. The pressure dependence of Tc is found negligible, different from the significant increase or a dome-shape seen in iso-structural layered diselenide superconductors. The high pressure magnetoresistance and Hall measurements suggest successive electronic structural changes with Fermi surface modifications at 6 GPa and 12GPa. From the observed negative magnetoresistance in this pressure range and absence of coexisting CDW and SC phases, we propose that intra-layer spin-fluctuation can play a role in the emergence of superconductivity in the high pressure phase.

Superconductivity in 5d transition metal Laves phase SrIr2

We report here the superconducting properties of a Laves phase superconductor SrIr2, which has a cubic MgCu2 structure. SrIr2 is a type-II superconductor, with a Tc of 5.9 K. The estimated superconducting parameters of lower critical field µ0Hc1 and upper critical field µ0Hc2, coherence length ξ(0), penetration depth λ(0) and Ginzburg–Landau (GL) parameter κ(0) are approximately µ0Hc1 = 101 Oe, µ0Hc2(0) = 5.9 T, ξ(0) = 7.47 nm, λ(0) = 237 nm, and κ(0) = 31.7, respectively. The specific-heat data indicate that SrIr2 is a strong-coupling superconductor because the value of ΔC/γTc is approximately 1.71, which is larger than the value of 1.43 that is expected from the BCS theory. The physical properties obtained in this study are explained well by theoretical calculations including spin–orbit coupling (SOC). This result indicates that the physical properties of SrIr2 are strongly affected by the presence of SOC.

Depth-resolved core level spectroscopy of noncentrosymmetric solid BiPd

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and topological phase of matter. Employing high resolution photoemission spectroscopy with photon energies ranging from hard x-ray to extreme ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derived the escape depths close to the extreme surface sensitivity and find that the photon energies used for high resolution measurements such as ARPES fall in the surface sensitive regime. In addition, we discover deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such paradigm shift in core level spectroscopy can be attributed to the absence of center of symmetry and spin-orbit interactions.

Topology-Controlled Phase Coherence and Quantum Fluctuations in Superconducting Nanowires

Superconducting properties of metallic nano-wires may strongly depend on specific experimental conditions. Here we consider a setup where superconducting phase fluctuations are restricted at one point inside the wire and equilibrium supercurrent flows along the wire segment of an arbitrary length $L$. Low temperature physics of this structure is essentially determined, on one hand, by smooth phase fluctuations and, on the other hand, by quantum phase slips. The zero temperature phase diagram is controlled by the wire cross section and consists of a truly superconducting phase and two different phases where superconductivity can be observed only at shorter length scales. One of the latter phases exhibits more robust short-scale superconductivity whereas another one demonstrates a power-law decay of the supercurrent with increasing $L$ already at relatively short scales.

Ab initio study of superconductivity and inhomogeneity in a Hg-based cuprate superconductor

Understanding physics of high-${T}_{c}$ cuprate superconductors remains one of the important problems in materials science. Though a number of diverse theories argue about the superconductivity and competing orders, ab initio and quantitative understanding is lacking. Here, we reproduce the experimental phase diagram of ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+y}$ by solving its ab initio low-energy effective Hamiltonian without adjustable parameters. It shows a superconducting phase in a wide range of hole density $\ensuremath{\delta}$, and its competition with charge period-4 plus spin period-8 stripe order near $\ensuremath{\delta}\ensuremath{\sim}0.1$, in agreement with experimental results including recent x-ray scattering. Then a crucial role of off-site interactions in stabilizing the superconductivity is elucidated with emphasis on charge fluctuations. It also clarifies the condensation energy mainly contributed from the on-site Coulomb interaction. The present achievement will enable deeper, predictable understanding on open issues of the high-${T}_{c}$ superconducting mechanism and promote ab initio studies on strongly correlated electrons beyond parametrized model studies.

Geometry-dependent effects in Majorana nanowires

Starting from the Bogolubov - de Gennes theory describing the induced p-wave superconductivity in the Majorana wire of an arbitrary shape, we predict a number of intriguing phenomena such as the geometry-dependent phase battery (or a phi-Josephson junction with the spontaneous superconducting phase difference) and generation of additional quasiparticle modes at the Fermi level with the spatial position tuned by the external magnetic field direction. This tuning can be used to extend the capabilities of the braiding protocols in Majorana networks.

Pairing correlations and eigenvalues of two-body density matrix in atomic nuclei

The quantum phases of superconductivity and superfluidity are characterized mathematically by the Yang concept of Off-Diagonal Long Range Order (ODLRO), related to the existence of a large eigenvalue of the density matrix. We analyse how the Yang criterion applies for various Hamiltonians commonly employed to describe superfluid-type correlations in atomic nuclei. For like-particle pairing Hamiltonians the behaviour of the largest eigenvalue of two-body density matrix shows a clear evidence for a transition between a normal to a paired phase. However, this is not the case for the isoscalar proton–neutron pairing interactions acting in self-conjugate nuclei.

Engineering nodal topological phases in Ising superconductors by magnetic superstructures

Recently it was discovered that superconductivity in transition metal dichalcogenides (TMDs) is strongly affected by an out-of-plane spin–orbit coupling. In addition, new techniques of fabricating 2d ferromagnets on van der Waals materials are rapidly emerging. Combining these breakthroughs, we propose a realization of nodal topological superconductivity in TMDs by fabricating nanostructured ferromagnets with an in-plane magnetization on the top surface. The proposed procedure does not require application of external magnetic fields and applies to monolayer and multilayer (bulk) systems. The signatures of the topological phase include Majorana flat bandsthat can be directly observed by scanning tunneling microscopy techniques. We concentrate on NbSe2 and argue that the proposed structures demonstrating the nodal topological phase can be realized within existing technology.

Quasiclassical circuit theory of contiguous disordered multiband superconductors

We consider a general problem of a Josephson contact between two multiband superconductors with coexisting superconducting and magnetic phases. As a particular example, we use the quasiclassical theory of superconductivity to study the properties of a Josephson contact between two disordered $s^{\pm}$-wave superconductors allowing for the coexistence between superconductivity and spin-density-wave orders. The intra- and inter-band scattering effects of disorder are treated within the self-consistent Born approximation. We calculate the spatial profile of the corresponding order parameters on both sides of the interface assuming that the interface has finite reflection coefficient and use our results to evaluate the local density of states at the interface as well as critical supercurrent through the junction as a function of phase or applied voltage. Our methods are particularly well suited for describing spatially inhomogeneous states of iron-based superconductors where controlled structural disorder can be created by an electron irradiation. We reveal the connection between our theory and the circuit-theory of Andreev reflection and extend it to superconducting junctions of arbitrary nature. Lastly, we outline directions for further developments in the context of proximity circuits of correlated electron systems.

Pressure-induced superconductivity in GeAs

We performed high-pressure Raman and resistance measurements on a two-dimensional monoclinic semiconductor GeAs. We discovered a superconductivity that emerges after the insulator-metal transition above 10 GPa, which is related to the structural transition. Our results indicate that the semiconducting monoclinic phase and rocksalt structure phase coexist above 10 GPa and structural transformation to the rocksalt phase is completed above $17\ensuremath{\sim}18\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. The maximum superconducting transition temperature ${T}_{c}$ is about 8 K close to the structural transition boundary, and the ${T}_{c}$ is gradually decreasing at higher pressures. Upon pressure release, the nonpolar rocksalt structure transforms to a polar tetragonal superconducting phase. Our first-principles calculations indicate that the relatively high ${T}_{c}$ in the rocksalt phase is mainly due to the enhancement of the electron-phonon coupling. These results show that superconductivity in this material strongly depends on its structure, achieving maximum ${T}_{c}$ in a higher-symmetry phase.

A simple tight-binding approach to topological superconductivity in monolayer MoS2

Monolayer molybdenum disulfide (MoS2) has a honeycomb crystal structure. Here, with considering the triangular sublattice of molybdenum atoms, a simple tight-binding Hamiltonian is introduced (derived) for studying the phase transition and topological superconductivity in MoS2 under uniaxial strain. It is shown that spin-singlet p + ip wave phase is a topological superconducting phase with nonzero Chern numbers. When the chemical potential is greater (smaller) than the spin–orbit coupling (SOC) strength, the Chern number is equal to four (two) and otherwise it is equal to zero. Also, the results show that, if the superconductivity energy gap is smaller than the SOC strength and the chemical potential is greater than the SOC strength, the zero energy Majorana states exist. Finally, we show that the topological superconducting phase is preserved under uniaxial strain.

Fractionalized pair density wave in the pseudogap phase of cuprate superconductors

The mysterious pseudo-gap (PG) phase of cuprate superconductors has been the subject of intense investigation over the last thirty years, but without a clear agreement about its origin. Owing to a recent observation in Raman spectroscopy, of a precursor in the charge channel, on top of the well known fact of a precursor in the superconducting channel, we present here a novel idea: the PG is formed through a Higgs mechanism, where two kinds of preformed pairs, in the particle-particle and particle-hole channels, become entangled through a freezing of their global phase. Remarkably, this entanglement is equivalent to fractionalizing a Cooper pair density wave (PDW) into its elementary parts; the particle-hole pair, giving rise to both density modulations and current modulations, and the particle-particle counterpart, leading to the formation of Cooper pairs. From this perspective, the "fractionalized PDW" becomes the central object around the formation of the pseudo-gap. The "locking" of phases between the charge and superconducting modes gives a unique explanation for the unusual global phase coherence of short-range charge modulations, observed below $T_{c}$ on phase sensitive scanning tunneling microscopy (STM). A simple microscopic model enables us to estimate the mean-field values of the precursor gaps in each channel and the PG energy scale, and to compare them to the values observed in Raman scattering spectroscopy. We also discuss the possibility of a multiplicity of orders in the PG phase and give an overview of the phase diagram.

Micron-Sized Constrictions Fabricated Using the Femtosecond Laser Technique on YBCO Thin Films

In this paper, I report on the fabrication of micron-sized constrictions in YBCO thin films using the femtosecond laser technique. The constrictions are S-shaped superconducting weak links fabricated on YBCO thin films. The constrictions show a measurable superconducting current flowing through them. Current–Voltage (I–V) characteristics were used to determine the critical current IC of the constrictions at 77 K. The width of the constrictions was determined at the narrowest point using SEM (scanning electron microscope). Three micron-sized constrictions were fabricated with a width of 2.88 μm, 1.72 μm, 1.69 μm and a length of 5 μm. Based on the width and length of these constrictions in relation to the coherence length (ξ) of the YBCO, these constrictions should conduct supercurrent by Abrikosov vortices. The SEM images show that the femtosecond laser heats the samples beyond the geometric boundary, damaging some of the superconductive phase of the YBCO constrictions. As a result, the constrictions conduct current by one-dimensional depairing.

Nematic Energy Scale and the Missing Electron Pocket in FeSe

Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving dxz, dyz, and dxy orbitals. The nematic energy scale between dxz and dyz bands reaches a maximum of 50 meV at the Brillouin zone corner. We are also able to track the dxz electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.

Properties of the H−T phase diagram of the 3−K phase in eutectic Sr2RuO4 -Ru: Evidence for chiral superconductivity

We use the Ginzburg-Landau theory for a chiral $p$-wave superconductor to describe the filamentary superconducting phase nucleating at temperatures higher than the bulk transition temperature near the interfaces between Ru metal and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ in eutectic ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$-Ru. The peculiar phase diagram of magnetic field versus temperature shows very distinct properties for different directions of the magnetic field. A clear feature of the magnetic fields in the basal plane of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, perpendicular to the chiral axis ($c$ axis), is the occurrence of a second superconducting transition with a new order parameter visible as a zero-bias anomaly in tunneling spectroscopy. For the field along the $c$ axis of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, two transitions merge, indicating that the field drives the second-order parameter through the paramagnetic coupling to a chiral supercurrent. The resulting phenomenology shows that the assumption of a chiral superconducting order parameter in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ yields an image reproducing the qualitative properties observed in the experiment. This discussion is qualitatively analogous to the corresponding chiral even- and odd-parity channels.

Pressure-induced phase transitions and superconductivity in magnesium carbides

Crystal structure prediction and in silico physical property observations guide experimental synthesis in high-pressure research. Here, we used magnesium carbides as a representative example of computational high-pressure studies. We predicted various compositions of Mg–C compounds up to 150 GPa and successfully reproduced previous experimental results. Interestingly, our proposed MgC2 at high pressure >7 GPa consists of extended carbon bonds, one-dimensional graphene layers, and Mg atomic layers, which provides a good platform to study superconductivity of metal intercalated graphene nano-ribbons. We found that this new phase of MgC2 could be recovered to ambient pressure and exhibited a strong electron-phonon coupling (EPC) strength of 0.6 whose corresponding superconductivity transition temperature reached 15 K. The EPC originated from the cooperation of the out-of-plane and the in-plane phonon modes. The geometry confinement and the hybridization between the Mg s and C pz orbitals significantly affect the coupling of phonon modes and electrons. These results show the importance of the high-pressure route to the synthesis of novel functional materials, which can promote the search for new phases of carbon-based superconductors.

Structural and Electronic Properties and Chemical Bonding in Layered 1111-Oxyarsenides LaRhAsO and LaIrAsO: AB Initio Simulation

Structural and electronic properties, Fermi surface topology, and chemical bonding features in layered 1111-oxyarsenides LaRhAsO and LaIrAsO are studied and compared using ab initio simulations. The nonmagnetic metal LaIrAsO is found to be weakly sensitive to both electronic and hole doping. However, the Rh-containing phase is predicted to exhibit weak band magnetism and can turn into a nonmagnetic state by hole doping. Therefore, the LaRhAsO oxyarsenide can be considered as a possible “electronic” analogue of the LaFeAsO compound, the basic phase of layered FeAs superconductors.

Micron-Sized Constrictions Fabricated Using the Femtosecond Laser Technique on YBCO Thin Films

In this paper, I report on the fabrication of micron-sized constrictions in YBCO thin films using the femtosecond laser technique. The constrictions are S-shaped superconducting weak links fabricated on YBCO thin films. The constrictions show a measurable superconducting current flowing through them. Current-voltage (I–V) characteristics were used to determine the critical current IC of the constrictions at 77 K. The width of the constrictions was determined at the narrowest point using SEM (scanning electron microscope). Three micron-sized constrictions were fabricated with a width of 2.88 μm, 1.72 μm, and 1.69 μm and a length of 5 μm. Based on the width and length of these constrictions in relation to the coherence length (ξ) of the YBCO, these constrictions should conduct supercurrent by Abrikosov vortices. The SEM images show that the femtosecond laser heats the samples beyond the geometric boundary, damaging some of the superconductive phases of the YBCO constrictions. As a result, the constrictions conduct current by one-dimensional depairing.

Surface Majorana flat bands in j = 3/2 superconductors with singlet-quintet mixing

Recent experiments [Science Advances 4 eaao4513 (2018)] have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors, e.g., YPtBi. Theories have suggested the topological nature of such nodal-line supercon-ductivity and proposed the existence of surface Majorana flat bands on the (111) surface of half-Heusler superconductors. Due to the divergent density of states of the surface Majorana flat bands, the surface order parameter and the surface impurity play essential roles in determining the surface properties. We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model. To be specific, we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing, classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3v point group, and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry. We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities. We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters. Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.

Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe2As2 in Heavily Electron-doped Regime

We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2.