Superconductivity In Single Crystals Research Articles

Magnetism and Superconductivity in the t–J Model of La3Ni2O7 Under Multiband Gutzwiller Approximation

The recent discovery of possible high temperature superconductivity in single crystals of La3Ni2O7 under pressure renews the interest in research on nickelates. The density functional theory calculations reveal that both d z 2 and d x 2–y 2 orbitals are active, which suggests a minimal two-orbital model to capture the low-energy physics of this system. In this work, we study a bilayer two-orbital t–J model within multiband Gutzwiller approximation, and discuss the magnetism as well as the superconductivity over a wide range of the hole doping. Owing to the inter-orbital super-exchange process between d z 2 and d x 2–y 2 orbitals, the induced ferromagnetic coupling within layers competes with the conventional antiferromagnetic coupling, and leads to complicated hole doping dependence for the magnetic properties in the system. With increasing hole doping, the system transfers to A-type antiferromagnetic state from the starting G-type antiferromagnetic (G-AFM) state. We also find the inter-layer superconducting pairing of d x 2–y 2 orbitals dominates due to the large hopping parameter of d z 2 along the vertical inter-layer bonds and significant Hund’s coupling between d z 2 and d x 2–y 2 orbitals. Meanwhile, the G-AFM state and superconductivity state can coexist in the low hole doping regime. To take account of the pressure, we also analyze the impacts of inter-layer hopping amplitude on the system properties.

Signatures of superconductivity near 80 K in a nickelate under high pressure.

Although high-transition-temperature (high-Tc) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown1-4. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with Tc above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La3Ni2O7 with maximum Tc of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the [Formula: see text] and [Formula: see text] orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the [Formula: see text] orbitals with the apical oxygen ions connecting the Ni-O bilayers. Thus, our discoveries provide not only important clues for the high-Tc superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-Tc superconductivity mechanism.

Strain tuning of nematicity and superconductivity in single crystals of FeSe

Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band structure calculations suggest that under strain the electron pockets develop a large in-plane anisotropy as compared with the hole pocket. Magnetotransport studies at low temperatures indicate that the mobility of the dominant carriers increases with tensile strain. Close to the critical temperature, all resistivity curves at constant strain cross in a single point, indicating a universal critical exponent linked to a strain-induced phase transition. Our results indicate that the superconducting state is enhanced under compressive strain and suppressed under tensile strain, in agreement with the trends observed in FeSe thin films and overdoped pnictides, whereas the nematic phase seems to be affected in the opposite way by the uniaxial strain. By comparing the enhanced superconductivity under strain of different systems, our results suggest that strain on its own cannot account for the enhanced high $T_c$ superconductivity of FeSe systems.

Superconductivity in the Z2 kagome metal KV3Sb5

Here we report the observation of bulk superconductivity in single crystals of the two-dimensional kagome metal KV$_3$Sb$_5$. Magnetic susceptibility, resistivity, and heat capacity measurements reveal superconductivity below $T_c = 0.93$K, and density functional theory (DFT) calculations further characterize the normal state as a $\mathbb{Z}_2$ topological metal. Our results demonstrate that the recent observation of superconductivity within the related kagome metal CsV$_3$Sb$_5$ is likely a common feature across the AV$_3$Sb$_5$ (A: K, Rb, Cs) family of compounds and establish them as a rich arena for studying the interplay between bulk superconductivity, topological surface states, and likely electronic density wave order in an exfoliable kagome lattice.

Emergent superconductivity by Re doping in type -II Weyl semimetal NiTe2

Recently topological semimetals emerge as a new platform to realise topological superconductivity. Here we report the emergent superconductivity in single-crystals of Re doped type-II Weyl semimetal NiTe2. The magnetic and transport measurements highlight that Re substitution in Ni-site induces superconductivity at a maximum temperature of 2.36 K. Hall effect and specific heat measurements indicate that Re substitution is doping hole and facilitates the emergence of superconductivity by phonon softening and enhancing the electron-phonon coupling.

CsV_{3}Sb_{5}: A Z_{2} Topological Kagome Metal with a Superconducting Ground State.

Recently discovered alongside its sister compounds KV_{3}Sb_{5} and RbV_{3}Sb_{5}, CsV_{3}Sb_{5} crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV_{3}Sb_{5}, demonstrating bulk superconductivity in single crystals with a T_{c}=2.5 K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV_{3}Sb_{5} as a Z_{2} topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E_{F}), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Superconductivity in single crystals of ZrP1.27Se0.73

Results are reported for single crystals of the PbFCl-type layered compound ${\mathrm{ZrP}}_{1.27}{\mathrm{Se}}_{0.73}$ that were produced using the iodine vapor transport method. Electrical transport, magnetization, and heat capacity measurements reveal disordered metallic behavior and the occurrence of bulk superconductivity, with a transition temperature $({T}_{c})$ of 7.1 K and an anisotropic orbitally limited upper critical field. $^{31}\mathrm{P}$ nuclear magnetic resonance measurements provide additional microscopic information, where the line shape, Knight shift, and spin-lattice relaxation data are consistent with the superconductivity originating from the corrugated Zr-P(2c)/Se plane. This suggests that the superconductivity first forms in the corrugated plane and the bulk superconductivity eventually occurs via coupling between the square planar layers. These data also show that either there are multiple superconducting gaps or there is a single gap that does not fully open across the entire Fermi surface. These results clarify the superconducting state in this material and will enable further measurements that require single-crystal specimens.

Two- and Three-Dimensional Superconducting Phases in the Weyl Semimetal TaP at Ambient Pressure

The motivation to search for signatures of superconductivity in Weyl semi-metals and other topological phases lies in their potential for hosting exotic phenomena such as nonzero-momentum pairing or the Majorana fermion, a viable candidate for the ultimate realization of a scalable quantum computer. Until now, however, all known reports of superconductivity in type-I Weyl semi-metals have arisen through surface contact with a sharp tip, focused ion-beam surface treatment or the application of high pressures. Here, we demonstrate the observation of superconductivity in single crystals, even an as-grown crystal, of the Weyl semi-metal tantalum phosphide (TaP), at ambient pressure. A superconducting transition temperature, T c , varying between 1.7 and 5.3 K, is observed in different samples, both as-grown and microscopic samples processed with focused ion beam (FIB) etching. Our data show that the superconductivity present in the as-grown crystal is inhomogeneous yet three-dimensional. For samples fabricated with FIB, we observe, in addition to the three-dimensional superconductivity, a second superconducting phase that resides on the sample surface. Through measurements of the characteristic fields as a function of temperature and angle, we are able to confirm the dimensionality of the two distinct superconducting phases.

Pairing from dynamically screened Coulomb repulsion in bismuth

Recently, Prakash et. al. have discovered bulk superconductivity in single crystals of bismuth, which is a semi metal with extremely low carrier density. At such low density, we argue that conventional electron-phonon coupling is too weak to be responsible for the binding of electrons into Cooper pairs. We study a dynamically screened Coulomb interaction with effective attraction generated on the scale of the collective plasma modes. We model the electronic states in bismuth to include three Dirac pockets with high velocity and one hole pocket with a significantly smaller velocity. We find a weak coupling instability, which is greatly enhanced by the presence of the hole pocket. Therefore, we argue that bismuth is the first material to exhibit superconductivity driven by retardation effects of Coulomb repulsion alone. By using realistic parameters for bismuth we find that the acoustic plasma mode does not play the central role in pairing. We also discuss a matrix element effect, resulting from the Dirac nature of the conduction band, which may affect $T_c$ in the $s$-wave channel without breaking time-reversal symmetry.

Weakly-correlated nodeless superconductivity in single crystals of Ca3Ir4Sn13 and Sr3Ir4Sn13 revealed by critical fields, Hall effect, and magnetoresistance measurements

We report a study of single crystal Ca3Ir4Sn13 (CIS) and Sr3Ir4Sn13 (SIS) by measuring the longitudinal and Hall resistivities, upper and lower critical fields and magnetoresistance, as well as the magnetization. The sign change in the Hall coefficient observed on both the CIS and SIS provides direct evidence for the Fermi surface reconstructing during the superlattice phase transition. Both materials are of current interest due to indications of superconductivity associated with charge-density-wave (CDW) ordering. Observations of the diamagnetic feature and the lower critical field Hc1(T) in both CIS and SIS can be realized by means of the nodeless single-gap BCS theory. In addition, a weak electronic correlation in both systems has been revealed by the small values of the spin exchange energy, upper critical field and Δ(0)/kBTc ratio, derived respectively from the normal-state Hall effect, resistive transition and temperature-dependent Hc1. It is noticeable that the magnetoresistance of SIS shows a rapid increase below T′ ∼ 40 K, following Kohler’s scaling rule. The results of the magnetic susceptibility and Hall coefficient also exhibit anomalous features near T′. With respect to these observations, this suggests that the existence of an additional phonon mode with energy of about 4.0 meV in SIS is responsible for the presence of lattice instability toward a phase transition.

Crystal growth and annealing study of fragile, non-bulk superconductivity in YFe2Ge2

We investigated the occurrence and nature of superconductivity in single crystals of grown out of Sn flux by employing X-ray diffraction, electrical resistivity and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as , by decanting the crystals from the molten Sn at and/or by annealing at temperatures between 550 and . We found that the samples with RRR 34 showed resistive signatures of superconductivity with the onset of the superconducting transition . RRR values vary between 35 and 65 with, on average, no systematic change in value, indicating that the systematic changes in RRR do not lead to comparable changes in . Specific heat measurements on samples that showed the clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain-stabilized superconductivity associated with small amounts of stressed (perhaps at twin boundaries or dislocations) or is a second crystallographic phase that is present at level below detection capability of conventional powder X-ray techniques.

Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba 1− x K x )Fe 2 As 2 ( x =0 and 0.45) and CaFe 2 As 2

We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe2As2, BaFe2As2, and (Ba0.55K0.45)Fe2As2, by means of measurements of electrical resistivity (rho) in the 1.8 - 300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe2As2 and BaFe2As2 (lightly doped with Sn) display structural phase transitions near 170 K and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba0.55K0.45)Fe2As2 is superconducting for T < 30 K. The effect of pressure on BaFe2As2 is to shift the onset of the crystallographic transformation down in temperature at the rate of about -1.04 K/kbar, while shifting the whole rho(T) curves downward, whereas its effect on superconducting (Ba0.55K0.45)Fe2As2 is to shift the onset of superconductivity to lower temperatures at the rate of about -0.21 K/kbar. The effect of pressure on CaFe2As2 is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P < 5 kbar. However, higher pressures bring about another phase transformation characterized by reduced resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (Hc2) data in (Ba0.55K0.45)Fe2As2 and CaFe2As2 are discussed.

Superconductivity without Fe or Ni in the phosphidesBaIr2P2andBaRh2P2

Heat capacity, resistivity, and magnetic susceptibility measurements confirm bulk superconductivity in single crystals of ${\text{BaIr}}_{2}{\text{P}}_{2}$ $({T}_{c}=2.1\text{ }\text{K})$ and ${\text{BaRh}}_{2}{\text{P}}_{2}$ $({T}_{c}=1.0\text{ }\text{K})$. These compounds form in the ${\text{ThCr}}_{2}{\text{Si}}_{2}$ (122) structure so they are isostructural to both the Ni and Fe pnictides but not isoelectronic to either of them. This illustrates the importance of structure for the occurrence of superconductivity in the 122 pnictides. Additionally, a comparison between these and other ternary phosphide superconductors suggests that the lack of interlayer P-P bonding favors superconductivity. These stoichiometric and ambient pressure superconductors offer an ideal playground to investigate the role of structure for the mechanism of superconductivity in the absence of magnetism.

Superconductivity in single crystals of LaFePO

Single crystals of the compound LaFePO were prepared using a flux growth technique athigh temperatures. Electrical resistivity measurements reveal metallic behaviorand a resistive transition to the superconducting state at a critical temperatureTc∼6.6 K. Magnetization measurements also show the onset of superconductivitynear 6 K. In contrast, specific heat measurements manifest no discontinuity atTc. These results lend support to the conclusion that the superconductivity isassociated with oxygen vacancies that alter the carrier concentration in asmall fraction of the sample, although superconductivity characterized by anunusually small gap value cannot be ruled out. Under applied magnetic fields,Tc is suppressed anisotropically for fields perpendicular and parallel to theab-plane, suggesting that the crystalline anisotropy strongly influences thesuperconducting state. Preliminary high pressure measurements show thatTc passes through a maximum of nearly 14 K at∼110 kbar, demonstrating that significantly higherTc values may be achieved in the phosphorus-based oxypnictides.

Discovery of plutonium-based superconductivity

The discovery of superconductivity in single crystals of PuCoGa5with transition temperature Tc = 18.5 K isdiscussed. The existing data lead to the speculation that the superconductivity in PuCoGa5may be unconventional. In such a scenario the properties of PuCoGa5would be intermediate between those of isostructural UCoGa5 andCeCoIn5,more heavily studied f-electron materials.

Retraction: Superconductivity in single crystals of the fullerene C70.

Nature 413, 831–833 (2001). This manuscript was, in part, the subject of an independent investigation conducted at the behest of Bell Laboratories, Lucent Technologies. The independent committee reviewed concerns related to the validity of data associated with the device measurements described in the paper.

Superconductivity in single crystals of the fullerene C70

The observation of superconductivity in doped C60 has attracted much attention, as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported for electron-doped C60 crystals, while a Tc of 52 K has been seen in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C60 raise the intriguing questions of whether conventional electron-phonon coupling alone can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes. There have, however, been no confirmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes. Here we report the observation of superconductivity in single crystals of electric-field-doped C70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C60 if the right charge density can be induced.

Superconductivity in single crystals of the nonperovskite-like cuprate Ba2Cu3O6−x

Spontaneous crystallization from a solution in a melt at a pressure of (0.8–1)×10−3 Pa was used to obtain single crystals of a new nonperovskite-like superconducting phase Ba2Cu3O6−x space group P ccm, a=13.065, b=20.654, and c=11.431 A). The superconducting properties of the crystals were investigated by modulated microwave absorption. The superconducting transition temperatures were 5 K (sample No. 1), 7 K and 13 K (sample No. 2). The superconductivity in the Ba2Cu3O6−x crystals is attributed to the presence of CuO2 chains of edge-sharing copper-oxygen squares. Nonperovskite-like Ba2Cu3O6−x crystals are a new class of one-dimensional (1D) superconductors.

Superconductivity of polycrystalline La 2− x− yNd ySr xCuO 4

Recent work on the charge stripe phase and its relationship to superconducting behavior observed in single crystals has motivated a reinvestigation of superconductivity in the La 2− x− y Nd y Sr x CuO 4 system. Literature values of superconducting transition temperature T c for nominally similar polycrystalline La 2− x− y Nd y Sr x CuO 4 are at variance [compare Crawford et al., Phys. Rev. B 44 (1991) 7749 and Buchner et al., Phys. Rev. Lett. 73 (1994) 1841]. This paper reports low magnetic field measurements of polycrystalline samples which support a broad range of superconductivity in good agreement with Crawford, and are consistent with the observed superconductivity in single crystals.

Effects of oxygen doping and structural distortion on the superconductivity of single crystals

Effects of oxygen doping and structural distortion on the superconductivity of (Bi2212) were studied by ac susceptibility and x-ray diffraction measurements for a group of high-quality single crystals. The crystals were annealed at 600 and under different oxygen pressures ranging from to 6080 Torr. It was found that the c-axis length showed a plateau at - Torr and decreased with increasing oxygen pressure above Torr, which strongly suggested that two stages of oxygen diffusion may be involved. The full width at half maximum (FWHM) of the 00 main reflection, which reflects structural distortion in the c direction, showed a peak around the optimized oxygen doping. This implies that the structural modification may be another factor that affects apart from oxygen content. On the other hand, the period of incommensurate modulation along the b direction, s, was found to have nothing to do with the oxygen content or superconductivity. Relationships between superconductivity, oxygen content and microstructure of the Bi2212 single crystal are discussed.