Occurrence Of Superconductivity Research Articles

Pressure-induced superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions

Despite extensive and systematic studies on pressure-induced phase transitions and superconductivity in sesqui-chalcogenide A2B3 (A=Sb, Bi; BS, Se, and Te) compounds, the detailed transporting properties of Bi2S3 have rarely been reported. Here, we discovered an unambiguous correlation between superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions. The rapid decrease in resistance below 10 GPa and the occurrence of superconductivity at approximately 25.5 GPa should be caused by the two unusual pressure-induced isostructural transitions. Subsequently, the superconducting transition temperature (TC) increased to around 7.3 K, which was accompanied by the amorphization of Bi2S3 above 43 GPa. Furthermore, divergence between the theoretical calculations and experimental measurements showed that the above phase transitions and pressure-induced charge transfer had a critical effect on the electronic properties of this compound. The results of this study indicated that high pressure could help to improve thermoelectric properties.

Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO2.

The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges1-4. The recent finding of superconductivity in layered nickelates raises similar interests5-8. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects9-11. Here, by moving to a substrate (LaAlO3)0.3(Sr2TaAlO6)0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd1-xSrxNiO2 essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides2,12 despite key distinctions-namely, the absence of an insulating parent compound5,6,9,10, multiband electronic structure13,14 and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides15,16. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.

Discovery of superconductivity in K-doped 2,2[formula omitted]-bipyridine

Organic compounds are always promising candidates of superconductors with high transition temperatures. The 2,2′-bipyridine solely composed by C, H, and N atoms is examined for this proposal. The presence of Meissner effect with a transition temperature of 7.2 K in this material upon potassium doping is demonstrated by the dc magnetic susceptibility measurements. The real part of the ac susceptibility exhibits the same transition temperature as that in dc magnetization, and a sharp peak appeared in the imaginary part indicates the formation of the weakly linked superconducting vortex current. The occurrence of superconductivity is further supported by the resistance drop at the transition together with its suppression by the applied magnetic fields. The superconducting phase is identified to be K3-2,2′-bipyridine with P21 space-group from the analysis of Raman scattering spectra and X-ray diffraction patterns as well as the first-principles calculations. This work not only opens an encouraging window for finding superconductivity after optoelectronics in 2,2′-bipyridine-based materials but also offers an example to realize superconductivity from conducting polymers and their derivatives.

Pressure-induced superconductivity in quasi-one-dimensional semimetal Ta2PdSe6

Here we report the discovery of pressure-induced superconductivity in quasi-one-dimensional ${\mathrm{Ta}}_{2}{\mathrm{PdSe}}_{6}$, through a combination of electrical transport, synchrotron x-ray diffraction, and theoretical calculations. Our transport measurements show that the superconductivity appears at a critical pressure ${P}_{\mathrm{c}}\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and is robust upon further compression up to 62.6 GPa. The estimated upper critical field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{c}2}(0)$ in the pressurized ${\mathrm{Ta}}_{2}{\mathrm{PdSe}}_{6}$ is much lower than the Pauli limiting field, in contrast to the case in its isostructural analogs ${M}_{2}{\mathrm{Pd}}_{\mathrm{x}}{X}_{5}$ $(M=\mathrm{Nb}, \mathrm{Ta}; X=\mathrm{S}, \mathrm{Se})$. Concomitant with the occurrence of superconductivity, anomalies in pressure-dependent transport properties are observed, including sign reversal of Hall coefficient, abnormally enhanced resistance, and dramatically suppressed magnetoresistance. Meanwhile, room-temperature synchrotron x-ray diffraction experiments reveal the stability of the pristine monoclinic structure (space group $C2$/$m$) upon compression. Combined with the density functional theory calculations, we argue that a pressure-induced Lifshitz transition could be the electronic origin of the emergent superconductivity in ${\mathrm{Ta}}_{2}{\mathrm{PdSe}}_{6}$.

Various manifestations of weak magnetism and superconductivity in inclination interfaces of Bi, Sb and Bi1-xSbx

The magnetic properties of the nano-width bicrystal interfaces (CIs) of Bi, Sb and 3D topological insulator Bi1xSbx (0.06 ≤ x ≤ 0.2) are studied in a temperature range of 1.6–300 K. These materials do not show superconductivity under normal rhombohedral conditions and are anomalous diamagnetics. At the same time, two superconducting phases with Tc ≤ 21 K and magnetic hysteresis loops against a diamagnetic background typical for strong type II superconductors are identified in Bi interfaces. At the CIs of Bi1xSbx (0.06 ≤ x ≤ 0.2), as well as Sb, a superconducting transition and a ferromagnetic hysteresis loop or a dual loop (superimposed ferromagnetic and superconducting loops) against a paramagnetic background are observed; they indicate the simultaneous occurrence of superconductivity and weak ferromagnetism, which is specific to 3D topological insulators. The revealed coexistence of superconductivity and weak magnetism at these CIs is of significant interest for the fundamental physics and future applications in quantum computing and spintronic devices.

Evidences for pressure-induced two-phase superconductivity and mixed structures of NiTe2 and NiTe in type-II Dirac semimetal NiTe2-x (x = 0.38 ± 0.09) single crystals

Bulk NiTe2 is a type-II Dirac semimetal with non-trivial Berry phases associated with the Dirac fermions. Theory suggests that monolayer NiTe2 is a two-gap superconductor, whereas experimental investigation of bulk NiTe1.98 for pressures (P) up to 71.2 GPa do not reveal any superconductivity. Here we report experimental evidences for pressure-induced two-phase superconductivity as well as mixed structures of NiTe2 and NiTe in Te-deficient NiTe2-x (x = 0.38 ± 0.09) single crystals. Hole-dominant multi-band superconductivity with the P3¯m1 hexagonal-symmetry structure of NiTe2 appears at P ≥ 0.5 GPa, whereas electron-dominant single-band superconductivity with the P2/m monoclinic-symmetry structure of NiTe emerges at 14.5 GPa < P < 18.4 GPa. The coexistence of hexagonal and monoclinic structures and two-phase superconductivity is accompanied by a zero Hall coefficient up to ∼ 40 GPa, and the second superconducting phase prevails above 40 GPa, reaching a maximum Tc = 7.8 K and persisting up to 52.8 GPa. Our findings suggest the critical role of Te-vacancies in the occurrence of superconductivity and potentially nontrivial topological properties in NiTe2-x.

Superconductivity mediated by polar modes in ferroelectric metals

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of Tc near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes.

Superconductivity Initiated by Electric Field in High-Temperature Superconductor at T &gt; Tc

The theory of occurrence of superconductivity in the plate of high-temperature superconductor under electric field vertical to it, when the carrier concentration lies outside of the region of existence of superconductivity at the temperature T < $$T_{{\text{c}}}^{{\max }}$$ , was developed. The calculation was carried out for the layer of lanthanum strontium cuprate under the fields of 10–1 V/nm ≥ E ≥ 10–2 V/nm at various temperatures and hole concentrations. It was demonstrated that the quasi-two-dimensional superconducting layer of several Angstrom in thickness occurs near surface of the plate, moreover thickness of this layer does not depend on magnitude of the field, and depends only on temperature and on hole concentration.

Possible Occurrence of Superconductivity by the π-flux Dirac String Formation Due to Spin-Twisting Itinerant Motion of Electrons

We show that the Rashba spin-orbit interaction causes spin-twisting itinerant motion of electrons in metals and realizes the quantized cyclotron orbits of conduction electrons without an external magnetic field. From the view point of the Berry connection, the cause of this quantization is the appearance of a non-trivial Berry connection A fic = − ℏ 2 e ∇ χ ( χ is an angular variable with period 2 π ) that generates π flux (in the units of ℏ = 1 , e = 1 , c = 1 ) inside the nodal singularities of the wave function (a “Dirac string”) along the centers of spin-twisting. Since it has been shown in our previous work that the collective mode of ∇ χ is stabilized by the electron-pairing and generates supercurrent, the π -flux Dirac string created by the spin-twisting itinerant motion will be stabilized by the electron-pairing and produce supercurrent.

Fourth-Order Ginzburg-Landau differential equation a la Fisher-Kolmogorov and quantum aspects of superconductivity

In this study, a 4th-order Ginzburg-Landau differential equation is obtained based on the generalization of the Heisenberg's uncertainty relation recently introduced in literature. This equation is similar to the Fisher-Kolmogorov equation which occurs in many physical models. The Lagrangian and the Hamiltonian of the 4th-prder Ginzburg-Landau equation were constructed and it was observed that the energy is a conserved quantity along its corresponding orbits. Two applications were discussed mainly the occurrence of superconductivity in a bulk superconductor through an external magnetic field and the microscopic formulation of BCS superconductivity. Several properties were discussed accordingly.

Two-gap superconductivity and topological surface states in TaOsSi

The occurrence of superconductivity in topological materials is considered as a promising route for realizing topological superconductors, a platform able to host the long-sought Majorana fermions in condensed matter. In this work, by using electrical transport and heat-capacity measurements, as well as first-principles band-structure calculations, we investigate the physical properties of TaOsSi, a superconductor with ${T}_{c}\ensuremath{\approx}5.8$ K. The behavior of both its upper critical field and low-temperature heat-capacity suggest the existence of two superconducting gaps. More strikingly, first-principles calculations reveal gapless topological surface states in the present material. The evolution of the electrical resistivity with pressure (up to 50 GPa) was also investigated, and a ``V-shaped'' diagram of ${T}_{c}$ vs $P$ was found. Overall, our data suggest that TaOsSi is a new system where multiband superconductivity and topological surface states coexist and, hence, it may serve as a possible candidate in the search for topological superconductivity.

Evolution of superconductivity and weak magnetism in inclination interfaces of Bi, Sb and Bi1-xSbx (0.07 ≤x ≤ 0.2) alloys

Using Quantum Design SQUID magnetometer, we studied the magnetic and superconducting properties of high quality inclination bicrystal interfaces of Bi, Sb and 3D topological insulator Bi1-xSbx (0.06 ≤ x ≤ 0.2). One or two superconducting phases with Tc ≤ 21 K and magnetic hysteresis loops on a diamagnetic background typical for strong type II superconductors were identified in Bi and some Bi-Sb nano-width interfaces. However, the other interfaces of Bi1-xSbx (0.06 ≤ x ≤ 0.2) as well as of Sb with a higher carrier density exhibit a superconducting transition and a ferromagnetic hysteresis loop or a dual loop (superimposed ferromagnetic and superconducting loops) against a paramagnetic background and thereby indicate the occurrence of superconductivity and weak ferromagnetism.

Probing the superconducting ground state of ZrIrSi: A muon spin rotation and relaxation study

The superconducting ground state of newly reported ZrIrSi is probed by means of $\mu$SR technique along with resistivity measurement. The occurrence of superconductivity at $T_\mathrm{C}$ = 1.7 K is confirmed by resistivity measurement. ZF-$\mu$SR study revealed that below $T_\mathrm{C}$, there is no spontaneous magnetic field in the superconducting state, indicates TRS is preserved in case of ZrIrSi. From TF-$\mu$SR measurement, we have estimated the superfluid density as a function of temperature, which is described by an isotropic $s-$wave model with a superconducting gap $2\Delta(0)/k_\mathrm{B}T_\mathrm{C}$ = 5.1, indicates the presence of strong spin-orbit coupling. {\it Ab-initio} electronic structure calculation indicates that there are four bands passing through the Fermi level, forming four Fermi surface pockets. We find that the low-energy bands are dominated by the $4d$-orbitals of transition metal Zr, with substantially lesser weight from the $5d$-orbitals of the Ir-atoms.

Lifshitz transition in new doped 112 superconductors

A detailed density functional theory based first principles electronic structure calculations on Ca1−xLaxFe1−yTMy(As1−pSbp)2 for various values of x, y, p and transition metals (TM = Co, Ni, Mn) are presented. We examine the possibility of Lifshitz like electronic topological transition in a variety of recently discovered doped 112 materials and demonstrate its possible connection with the occurrence of superconductivity in these materials. We show that La doping at Ca site, Co or Ni doping at Fe site causes electron doping whereas Mn doping at Fe site causes hole doping. As the level of electron doping is enhanced either through La or TM (= Ni, Co) doping the hole bands drift below the Fermi level causing Lifshitz transition. At the verge of Lifshitz transition, a merger of top of hole bands in the close vicinity of Fermi level i.e, singular band structure are found that occurs at around optimal doping. We show that the hole bands move from above the Fermi level to below it with La doping, and at x = 0.25, all the hole bands move below the Fermi level causing a topological modification in the hole Fermi surfaces, creating disconnected Fermi surfaces at the Γ point where superconductivity is predicted to cease - this is in agreement with experimental observations. We also predict occurrence of Lifshitz transition in more recently discovered Ca1−xLaxFe1−yNiy As2 for various values of doping concentration x and y. In particular for x = 0.18, 0.24 and y = 0.005, 0.010, 0.015, 0.020, the system is on the verge of Lifshitz transition. While the results of Co doping is similar to that of Ni doping, Mn doping has adverse effect on superconductivity causing its absence. Unique modifications in the electronic structures compared to other family of Fe based superconductors are obtained through Sb doping at As belonging to FeAs layers as well as that belonging to chains in between the layers. While Sb doping at As of FeAs layer causes enhancement of electronic Fermi arcs around the high symmetry X point of the Brillouin zone, the same on the chain As atom suppresses its formation at X. Contribution of chain As atoms and their role in Lifshitz transition are also demonstrated. Enhanced contribution to density of states from chain atoms in comparison to that of the layers signify possible role of chain atoms in contributing to superconductivity in these materials.

Observation of interface superconductivity in a SnSe2 /epitaxial graphene van der Waals heterostructure

We report on the direct observation of interface superconductivity in single-unit-cell SnSe2 films grown on graphitized SiC(0001) substrate by means of van der Waals epitaxy. Tunneling spectrum in the superconducting state reveals rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the presence of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of SnSe2 and graphene. Our finding opens up novel strategies to hunt for and understand interface superconductivity based on van der Waals heterostructures.

Superconductivity and Phase Stability of Potassium-Intercalated p-Quaterphenyl.

To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by an annealing or just a pestling process. The Meissner effect with critical temperatures ranging from 3.5 to 120 K is found by the magnetic susceptibility measurements in doped samples. The primary superconducting phase with a critical temperature of 7.2 K can be duplicated in the annealed and pestled samples. The charge transfer from metal to molecule is confirmed from the Raman scattering measurements. The X-ray diffraction analysis suggests that the low-temperature superconducting phase is due to the two-electron doping, whereas the high-temperature one corresponds to the high doping content. The occurrence of superconductivity in potassium-doped p-quaterphenyl supports the chain link organic molecules as promising candidates for high-temperature superconductors. This work also provides a simple method for synthesizing organic superconductors by pestling without annealing.

Superconductivity in LaPd2Bi2 with CaBe2Ge2-type structure

Here we report the synthesis and superconductivity of a novel ternary compound LaPd2Bi2. Shiny plate-like single crystals of LaPd2Bi2 were first synthesized by high-temperature solution method with PdBi flux. X-ray diffraction analysis indicates that LaPd2Bi2 belongs to the primitive tetragonal CaBe2Ge2-type structure with the space group P4/nmm (No. 129), and the refined lattice parameters are a = 4.717(2) {\AA}, c = 9.957(3) {\AA}. Electrical resistivity and magnetic susceptibility measurements reveal that LaPd2Bi2 undergoes a superconducting transition at 2.83 K and exhibits the characteristics of type-II superconductivity. The discovery of superconductivity in LaPd2Bi2 with CaBe2Ge2-type structure may help to further understand the possible relationship between the occurrence of superconductivity and the crystal structures in 122-type materials.

Anomalous 125Te Nuclear Spin Relaxation Coincident with Charge Kondo Behavior in Superconducting Pb1−xTlxTe

We report a ^{125}Te-NMR study of single-crystalline Pb_{1-x}Tl_{x}Te for x= 0 (undoped non-superconducting "parent"), 0.35 at% (doped but on the borderline of superconductivity), and 1.0 at% (superconducting, with a critical temperature Tc ~ 1.0 K). The Knight shift in the normal state is enhanced as x increases, corresponding to an increase in the average density of states (DOS). The NMR line-width also increases significantly with increasing x, indicative of a strong spatial variation in the local DOS surrounding each Tl dopant. Remarkably, for the superconducting composition, the ^{125}Te nuclear spin relaxation rate (1/T1T) for Te ions that are close to the Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~10 K. This temperature coincides with the temperature below which the normal state resistivity experiences an upturn. Such a simultaneous upturn in both the resistivity and (1/T1T) is highly anomalous, and puts Pb_{1-x}Tl_{x}Te in a distinct new class of doped semiconductors. These observations provide microscopic evidence for dynamical charge fluctuations in the absence of paramagnetism, and are consistent with expectations for charge Kondo behavior associated with the Tl dopant ions. In contrast, such anomalies were not detected in the non-superconducting samples (x=0 and 0.35 at%), suggesting a connection between dynamical valence fluctuations and the occurrence of superconductivity in Pb_{1-x}Tl_{x}Te.

Spin quenching assisted by a strongly anisotropic compression behavior in MnP

We studied the crystal structure and spin state of MnP under high pressure with synchrotron x-ray diffraction and x-ray emission spectroscopy (XES). MnP has an exceedingly strong anisotropy in compressibility, with the primary compressible direction along the b axis of the Pnma structure. XES reveals a pressure-driven quenching of the spin state in MnP. First-principles calculations suggest that the strongly anisotropic compression behavior significantly enhances the dispersion of the Mn d-orbitals and the splitting of the d-orbital levels compared to the hypothetical isotropic compression behavior. Thus, we propose spin quenching results mainly from the significant enhancement of the itinerancy of d electrons and partly from spin rearrangement occurring in the split d-orbital levels near the Fermi level. This explains the fast suppression of magnetic ordering in MnP under high pressure. The spin quenching lags behind the occurrence of superconductivity at ∼8 GPa implying that spin fluctuations govern the electron pairing for superconductivity.

Absence of superconductivity in LiPdHx

Theoretical calculations about LiPdH predict that it may be a superconductor. Using the high-pressure synthesis technique up to 3 GPa, we have successfully fabricated the LiPdH compound. The sample looks quite pure judging from the refinement of the X-ray diffraction pattern. However, no superconductivity has been detected down to about 2 K. The specific heat measurement shows a small Sommerfeld constant = 1.22 mJ mol K, which is a bit smaller than the theoretically predicted value. The magnetic susceptibility measured at 1 Tesla reveals a Curie–Weiss law behaviour in the low-temperature region with an estimate of the magnetic moment of 0.08 /Pd. The temperature dependence of resistivity has also been measured up to 25.2 GPa, but superconductivity is still not observed above 2 K. The large residual resistivity and small residual-resistivity ratio suggest that the quasiparticle scattering by impurities is quite strong, which might be induced by the hydrogen deficiency, and intimately hinders the occurrence of superconductivity in LiPdH.