Unconventional Superconducting State Research Articles

Coexistence of antiferromagnetism and unconventional superconductivity in a quasi-one-dimensional flat-band system: Creutz lattice

We study the coexistence of antiferromagnetism and unconventional superconductivity on the Creutz lattice which shows strictly flat bands in the noninteracting regime. The famous renormalized mean-field theory is used to deal with strong electron–electron repulsive Hubbard interaction in the effective low-energy t–J model, the superfluid weight of the unconventional superconducting state has been calculated via the linear response theory. An unconventional superconducting state with both spin-singlet and staggered spin-triplet pairs emerges beyond a critical antiferromagnetic coupling interaction, while antiferromagnetism accompanies this state. The superconducting state with only spin-singlet pairs is dominant with paramagnetic phase. The A phase is analogous to the pseudogap phase, which shows that electrons go to form pairs but do not cause a supercurrent. We also show the superfluid behavior of the unconventional superconducting state and its critical temperature. It is proven directly that the flat band can effectively raise the critical temperature of superconductivity. It is implementable to simulate and control strongly-correlated electrons’ behavior on the Creutz lattice in the ultracold atoms experiment or other artificial structures. Our results may help the understanding of the interplay between unconventional superconductivity and magnetism.

Charge correlations suppress unconventional pairing in the Holstein model

In a recent work by Schrodi $\textit{et al}$. [Phys. Rev. B. $\textbf{104}$, L140506 (2021)], the authors find an unconventional superconducting state with a sign-changing order parameter using the Migdal-Eliashberg theory, including the first vertex correction. This unconventional solution arises despite using an isotropic bare electron-phonon coupling in the Hamiltonian. We examine this claim using hybrid quantum Monte Carlo for a single-band Holstein model with a cuprate-like noninteracting band structure and identical parameters to Schrodi $\textit{et al}$.. Our Monte Carlo results for these parameters suggest that unconventional pairing correlations do not exceed their noninteracting values at any carrier concentration we have checked. Instead, strong charge-density-wave correlations persist at the lowest accessible temperatures for dilute and nearly half-filled bands. Lastly, we present arguments for how vertex-corrected Migdal-Eliashberg calculation schemes can lead to uncontrolled results in the presence of Fermi surface nesting.

Chemical and Physical Properties of YHg3 and LuHg3.

Amalgams have played an important role in fundamental and applied solid-state chemistry and physics because of the diversity of crystallographic features and properties that they have to offer. Moreover, their peculiar chemical properties can sometimes give rise to unconventional superconducting or magnetic ground states. In the current work, we present an in-depth analysis of single crystals of YHg3 and LuHg3 (Mg3Cd structure type, space group P63/mmc). Both compounds show superconductivity below Tc = 1 ± 0.1 K (YHg3) and Tc = 1.2 ± 0.1 K (LuHg3). Given the high air-sensitivity and toxicity of these compounds, this study was only possible using a number of dedicated experimental techniques.

Quantum oscillations and quasiparticle properties of thin film Sr2RuO4

We measure the Shubnikov--de Haas effect in thin-film ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ grown on an $({\mathrm{LaAlO}}_{3}{)}_{0.29}\text{\ensuremath{-}}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_{3}{)}_{0.71}$ substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, and that the quasiparticle lifetime is consistent with the ${T}_{\mathrm{c}}$ of comparably clean, single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. Unlike single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is $1.3\ifmmode\pm\else\textpm\fi{}0.1$ times longer than the quantum lifetime. This may suggest that extended (rather than point) defects could be the dominant source of quasiparticle scattering in these films. To test this hypothesis, we perform cross-sectional scanning transmission electron microscopy and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ through the fabrication of devices---such as planar tunnel junctions and superconducting quantum interference devices.

Linkage between scattering rates and superconductivity in doped ferropnictides

We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.

Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb0.25Bi2Se3

Recently, the niobium (Nb)-doped topological insulator Bi_2Se_3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi_2Se_3 layers, has been shown to exhibit both superconductivity with T_c = 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature-dependent of effective magnetic penetration depth Lambda_eff(T) in the layered topological superconductor candidate Nb_0.25Bi_2Se_3. The exponential temperature dependence of lambda_(eff)^(-2)(T) at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature T_c = 2.9 K and the gap to T_c ratio 2Delta/k_BT_c = 3.95(19). We also revealed that the ratio T_c/lambda_(eff)^(-2) is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field muSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically non-trivial electronic structure of Nb_0.25Bi_2Se_3.

Unconventional chiral d-wave superconducting state in strained graphene**Project supported by the National Natural Science Foundation of China (Grant Nos. 11804213 and 11605109).

We consider a highly unconventional superconducting state with chiral d-wave symmetry in doped graphene under strain with the Gutzwiller–RVB method in the momentum space. It is shown that flat bands emerge in the normal state for reasonable strain. As a result, the superconducting critical temperature is found to be linearly proportional to the strength of the electron–electron interaction. Furthermore, the chiral d-wave superconducting state is shown with coexistence of the charge density wave and the pair density wave. There are different coexisting states with those orders under different doping levels.

Possible quantum critical behavior revealed by the critical current density of hole doped high-Tc cuprates in comparison to heavy fermion superconductors

The superconducting critical current density, Jc, in hole doped cuprates show strong dependence on the doped hole content, p, within the copper oxide plane(s). The doping dependent Jc mainly exhibits the variation of the intrinsic depairing critical current density as p is varied. Jc(p) tends to peak at p ~ 0.185 in copper oxide superconductors. This particular value of the hole content, often termed as the critical hole concentration, has several features putative to a quantum critical point (QCP). Very recently, the pressure dependences of the superconducting transition temperature (Tc) and the critical current (Ic) in pure CeRhIn5 and Sn doped CeRhIn5 heavy fermion compounds have been reported (Nature Communications (2018) 9:44, https://doi.org/10.1038/s41467-018-02899-5). The critical pressure demarcates an antiferromagnetic quantum critical point where both Tc and Ic are maximized. We have compared and contrasted this behavior with those found for Y1−xCaxBa2Cu3O7−δ in this brief communication. The resemblance of the systematic behavior of the critical current with pressure and hole content between heavy fermion systems and hole doped cuprates is significant. This adds to the circumstantial evidence that quantum critical physics probably plays a notable role behind the unconventional normal and superconducting state properties of copper oxide superconductors.

Fate of a strongly correlated d -wave superconductor in a Zeeman field: The Fulde-Ferrel-Larkin-Ovchinnikov perspective

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase is an unconventional superconducting state found under the influence of strong Zeeman field. This phase is identified by finite center-of-mass momenta in the Cooper pairs, causing the pairing amplitude to oscillate in real space. Repulsive correlations, on the other hand, smear out spatial inhomogeneities in d-wave superconductors. We investigate the FFLO state in a strongly correlated d-wave superconductor within a consolidated framework of Hartree-Fock-Bogoliubov theory and Gutzwiller approximation. We find that the profound effects of strong correlations lie in shifting the BCS-FFLO phase boundary towards a lower Zeeman field and thereby enlarging the window of the FFLO phase. In the FFLO state, our calculation features a sharp mid-gap peak in the density of states, indicating the formation of strongly localized Andreev bound states. We also find that the signatures of the FFLO phase survive even in the presence of an additional translational symmetry breaking competing order in the ground state. This is demonstrated by considering a broken symmetry ground state with a simultaneous presence of the d-wave superconducting order and a spin-density wave order, often found in unconventional superconductors.

UTe2: A New Spin-Triplet Pairing Superconductor

Superconducting ferromagnets provide a fruitful playground to study exotic forms of Cooper pairing. Recently, a new superconductor at the verge of ferromagnetic order has been reported: UTe2. Now researchers from Tohoku University, Kobe University and CEA/Grenoble have joined efforts and uncover an unconventional superconducting state.

Proximity-Induced Superconducting States of Magnetically Doped 3D Topological Insulators with High Bulk Insulation

We studied magnetized topological insulator/superconductor junctions with the expectation of unconventional superconductive states holding Majorana fermions induced by superconductive proximity effects on the surface states of magnetized topological insulators (TIs), attached by conventional superconductors. We introduced Fe-doped BiSbTe2Se as an ideal magnetic TI and used the developed junction fabrication process to access the proximity-induced surface superconducting states. The bulk single crystals of the Fe-doped TI showed excellent bulk-insulating properties and ferromagnetism simultaneously at a low temperature. Meanwhile, the fabricated junctions also showed an insulating behavior above 100 K, as well as metallic conduction at a low temperature, which reflects bulk carrier freezing. In addition, we observed a proximity-induced gap structure in the conductance spectra. These results indicate that the junctions using the established materials and process are preferable to observe unconventional superconducting states which are induced via the surface channels of the magnetized TI. We believe that the developed process can be applied for the fabrication of complicated junctions and suites for braiding operations.

Robust parity-mixed superconductivity in disordered monolayer transition metal dichalcogenides

Monolayer NbSe$_2$ is a nodal topological Ising superconductor at magnetic in-plane fields exceeding the Pauli limit, with nodal points strictly on high symmetry lines in the Brillouin zone. Here, we use a combined numerical and group-theoretical approach in real-space to characterize the unconventional superconducting state in monolayer transition metal dichalcogenides. Even with a conventional pairing interaction, the superconducting state is intrinsically parity-mixed and robust against on-site disorder. The interplay between the Zeeman magnetic field, strong spin-orbit interaction, and electronic orbital content confer the unique superconducting and topological properties. The discussion also extends to strongly hole-doped MoS$_2$ and its relatives.

Singlet-quintet mixing in spin-orbit coupled superconductors with j=32 fermions

In non-centrosymmetric superconductors, spin-orbit coupling can induce an unconventional superconducting state with a mixture of s-wave spin-singlet and p-wave spin-triplet channels, which leads to a variety of exotic phenomena, including anisotropic upper critical field, magnetoelectric effect, topological superconductivity, et al. It is commonly thought that inversion symmetry breaking is substantial for pairing-mixed superconducting states. In this work, we theoretically propose that a new type of pairing-mixed state, namely the mixture of s-wave spin-singlet and d-wave spin-quintet channels, can be induced by spin-orbit coupling even in the presence of inversion symmetry when electrons effectively carry "spin-3/2" in superconductors. As a physical consequence of the singlet-quintet pairing mixing, topological nodal-line superconductivity is found in such system and gives rise to flat surface Majorana bands. Our work provides a possible explanation of unconventional superconducting behaviors observed in superconducting half-Heusler compounds.

Asymmetric d-wave superconducting topological insulator in proximity with a magnetic order

In the framework of the Dirac–Bogoliubov–de Gennes formalism, we investigate the transport properties in the surface of a 3-dimensional topological insulator-based hybrid structure, where the ferromagnetic and superconducting orders are simultaneously induced to the surface states via the proximity effect. The superconductor gap is taken to be spin-singlet d-wave symmetry. The asymmetric role of this gap respect to the electron–hole exchange, in one hand, affects the topological insulator superconducting binding excitations and, on the other hand, gives rise to forming distinct Majorana bound states at the ferromagnet/superconductor interface. We propose a topological insulator N/F/FS junction and proceed to clarify the role of d-wave asymmetry pairing in the resulting subgap and overgap tunneling conductance. The perpendicular component of magnetizations in F and FS regions can be at the parallel and antiparallel configurations leading to capture the experimentally important magnetoresistance (MR) of junction. It is found that the zero-bias conductance is strongly sensitive to the magnitude of magnetization in FS region mzfs and orbital rotated angle α of superconductor gap. The negative MR only occurs in zero orbital rotated angle. This result can pave the way to distinguish the unconventional superconducting state in the relating topological insulator hybrid structures.

Competing magnetic orders in the superconducting state of heavy-fermion CeRhIn5

Applied pressure drives the heavy-fermion antiferromagnet CeRhIn5 toward a quantum critical point that becomes hidden by a dome of unconventional superconductivity. Magnetic fields suppress this superconducting dome, unveiling the quantum phase transition of local character. Here, we show that [Formula: see text] magnetic substitution at the Ce site in CeRhIn5, either by Nd or Gd, induces a zero-field magnetic instability inside the superconducting state. This magnetic state not only should have a different ordering vector than the high-field local-moment magnetic state, but it also competes with the latter, suggesting that a spin-density-wave phase is stabilized in zero field by Nd and Gd impurities, similarly to the case of Ce0.95Nd0.05CoIn5 Supported by model calculations, we attribute this spin-density wave instability to a magnetic-impurity-driven condensation of the spin excitons that form inside the unconventional superconducting state.

Full-gap superconductivity in noncentrosymmetric Re6Zr,Re27Zr5 , and Re24Zr5

Non-centrosymmetric superconductor Re$_6$Zr has attracted much interest, for its possible unconventional superconducting state with time reversal symmetry broken. Here we report the $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements on Re$_6$Zr ($T_c$ = 6.72 K) and the isostructural compounds Re$_{27}$Zr$_{5}$ ($T_c$ = 6.53 K) and Re$_{24}$Zr$_{5}$ ($T_c$ = 5.00 K). The nuclear spin-lattice relaxation rate $1/T_1$ shows a coherence peak below $T_c$ and decreases exponentially at low temperatures in all three samples. The superconducting gap $\Delta$ derived from the $1/T_1$ data is $2\Delta =3.58 $ $k_BT_c$, 3.55 $k_BT_c$, and 3.51 $k_BT_c$ for Re$_6$Zr ,Re$_{27}$Zr$_{5}$, and Re$_{24}$Zr$_{5}$, respectively, which is close to the value of 3.53 $k_BT_c$ expected for weak-coupling superconductivity. These data suggest conventional $s$-wave superconductivity with a fully-opened gap in this series of compounds.

Unconventional Disorder Effects in Correlated Superconductors.

We study the effects of disorder on unconventional superconductors in the presence of correlations, and explore a novel correlated disorder paradigm dominated by strong deviations from standard Abrikosov-Gor'kov theory due to generation of local bound states and cooperative impurity behavior driven by Coulomb interactions. Specifically we explain under which circumstances magnetic disorder acts as a strong poison destroying high-T_{c} superconductivity at the sub-1% level, and when nonmagnetic disorder, counterintuitively, hardly affects the unconventional superconducting state while concomitantly inducing an inhomogeneous full-volume magnetic phase. Recent experimental studies of Fe-based superconductors have discovered that such unusual disorder behavior seems to be indeed present in those systems.

High-pressure study of the basal-plane anisotropy of the upper critical field of the topological superconductor SrxBi2Se3

We report a high-pressure transport study of the upper-critical field, $B_{c2}(T)$, of the topological superconductor Sr$_{0.15}$Bi$_2$Se$_3$ ($T_c = 3.0$ K). $B_{c2}(T)$ was measured for magnetic fields directed along two orthogonal directions, $a$ and $a^*$, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure $p_c \sim 3.5$ GPa, the pronounced two-fold basal-plane anisotropy $B_{c2}^a/B_{c2}^{a^*} = 3.2$ at $T=0.3$ K, recently reported at ambient pressure (Pan et al., 2016), is reinforced and attains a value of $\sim 5$ at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure.

Superconductivity in doped Dirac semimetals

We theoretically study intrinsic superconductivity in doped Dirac semimetals. Dirac semimetals host bulk Dirac points, which are formed by doubly degenerate bands, so the Hamiltonian is described by a $4 \times 4$ matrix and six types of $k$-independent pair potentials are allowed by the Fermi-Dirac statistics. We show that the unique spin-orbit coupling leads to characteristic superconducting gap structures and $d$ vectors on the Fermi surface and the electron-electron interaction between intra and interorbitals gives a novel phase diagram of superconductivity. It is found that when the inter-orbital attraction is dominant, an unconventional superconducting state with point nodes appears. To verify the experimental signature of possible superconducting states, we calculate the temperature dependence of bulk physical properties such as electronic specific heat and spin susceptibility and surface state. In the unconventional superconducting phase, either dispersive or flat Andreev bound states appear between point nodes, which leads to double peaks or single peak in the surface density of states, respectively. As a result, possible superconducting states can be distinguished by combining bulk and surface measurements.

Differential conductance and defect states in the heavy-fermion superconductorCeCoIn5

We demonstrate that the electronic bandstructure extracted from quasi-particle interference spectroscopy [Nat. Phys. 9, 468 (2013)] and the theoretically computed form of the superconducting gaps [Proc. Nat. Acad. Sci. 111, 11663 (2014)] can be used to understand the $dI/dV$ lineshape measured in the normal and superconducting state of CeCoIn$_5$ [Nat. Phys. 9, 474 92103)]. In particular, the $dI/dV$ lineshape, and the spatial structure of defect-induced impurity states, reflects the existence of multiple superconducting gaps of $d_{x^2-y^2}$-symmetry. These results strongly support a recently proposed microscopic origin of the unconventional superconducting state.