Giant nonreciprocal transport effect in noncentrosymmetric superconductors is studied both theoretically and experimentally.

Multiorbital Hubbard models are shown to exhibit a spatially isotropic spin-triplet superconducting phase, where equal-spin electrons in different local orbitals are paired. This superconducting state is stabilized in the spin-freezing crossover regime, where local moments emerge in the metal phase, and the pairing is substantially assisted by spin anisotropy. The phase diagram features a superconducting dome below a non-Fermi-liquid metallic region and next to a magnetically ordered phase. We suggest that this type of fluctuating-moment-induced superconductivity, which is not originating from fluctuations near a quantum critical point, may be realized in spin-triplet superconductors such as strontium ruthenates and uranium compounds.

The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many body physics. The conventional superconductors are insensitive to dilute nonmagnetic impurities, known as the Anderson's theorem 1 .Destruction of superconductivity and even superconductor-insulator transitions 2-10 occur in the regime of strong disorder. Hence disorder-enhanced superconductivity is rare and has only been observed in some alloys or granular states 11-17 . Because of the entanglement of various effects, the mechanism of enhancement is still under debate.Here we report well-controlled disorder effect in the recently discovered monolayer

The alkali-doped fullerides A3C60 are half-filled three-orbital Hubbard systems which exhibit an unconventional superconducting phase next to a Mott insulator. While the pairing is understood to arise from an effectively negative Hund coupling, the highly unusual Jahn-Teller metal near the Mott transition, featuring both localized and itinerant electrons, has not been understood. This property is consistently explained by a previously unrecognized phenomenon: the spontaneous transition of multiorbital systems with negative Hund coupling into an orbital-selective Mott state. This symmetry-broken state, which has no ordinary orbital moment, is characterized by an orbitaldependent two-body operator (the double occupancy) or an orbital-dependent kinetic energy, and may be regarded as a diagonal-order version of odd-frequency superconductivity. We propose that the recently discovered Jahn-Teller metal phase of RbxCs3−xC60 is an experimental realization of this novel state of matter.The appearance of long-range order by spontaneous symmetry breaking (SSB) is a fundamental and widely studied concept in physics. In condensed matter systems, the ordered state is typically characterized by an order parameter measuring the charge, magnetic moment, orbital angular momentum, or the pair amplitude in a superconductor. There may however exist more complex types of ordering phenomena. Here we demonstrate that in a certain class of multi-orbital systems one observes an orbital symmetry breaking into a state without conventional order parameter, but with an orbital-dependent double occupancy (a composite order parameter) and an orbital-dependent kinetic energy. The resulting ordered phase is a spontaneous orbital-selective Mott (SOSM) state, in which itinerant and localized electrons coexist. As this state combines properties of the metal (connected to the weak-interaction limit) and Mott insulator (connected to the strong-interaction limit) it cannot be detected by perturbative methods from either limits, and its study requires the use of sophisticated techniques. The ordering phenomenon can be discussed in terms of a symmetry-breaking field or order parameter with odd time (frequency) dependence, and is hence related to the concept of odd-frequency superconductivity [1-4].We will argue that this unconventional SOSM state is realized in alkali-doped fullerides [5][6][7][8][9][10][11][12][13], which are promising candidates for diagonal odd-frequency orders. Several compounds in this class of materials can be regarded as strongly correlated systems since a Mott transition occurs as a function of pressure. In the case of an fcc lattice [12], the Mott insulator stays paramagnetic in a wide range of temperature due to geometrical frustration, while it is antiferromagnetically ordered in the case of a bcc lattice, which is bipartite [11]. With increasing pressure, the system turns into a paramagnetic metal and is unstable to superconductivity below a maximum T c ≃ 38 K [11]. Recently, a so-called Jahn-Teller metal (JTM) state has bee...

Superconductivity with a remarkably high Tc has recently been found in Sr-doped NdNiO2 thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. Sr0.2Nd0.8NiO2 falls into a parameter regime influenced by two spin-freezing crossovers, one related to the emergent multi-orbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.Nickelate analogs of the cuprates such as LaNiO 2 had been theoretically proposed more than 20 years ago [1], but only very recently has superconductivity been found in Sr-doped NdNiO 2 thin films [2]. This exciting discovery offers a new platform to study unconventional superconductivity and may provide new insights into the pairing mechanism in cuprate-like systems. Several theoretical investigations on the new compound have already been conducted [4][5][6][7][8]. They essentially confirm the results of earlier bandstructure calculations [3], which suggest an intrinsic hole-doping of the Ni 3d x 2 −y 2 band by Nd 5d pockets. The presence of the 5d states at the Fermi surface led to speculation about an important role of the hybridization between the strongly correlated 3d and more extended 5d states, and possible analogies to heavy-fermion superconductivity [2,6]. A detailed abinitio study however suggests an almost perfect decoupling between the Ni 3d x 2 −y 2 states and those in the Nd layer [8]. The close analogy to the cuprates and the relatively high T c ∼ 10 K, which cannot be explained by a phonon-mediated pairing mechanism [8], suggests unconventional superconductivity with most likely a d-wave order parameter [5,7].Two potentially relevant differences between the nickelate and cuprate superconductors have however been pointed out [3,4]. One is the substantially smaller antiferromagnetic exchange coupling resulting from the larger splitting between the Ni 3d and O 2p bands in the nickelates. This appears to pose a problem if one tries to explain high-T c superconductivity as a pairing induced by antiferromagnetic spin fluctuations. The other difference is that the nickelate compound is a (doped) Mott insulator, where the doped holes end up on the Ni sites, whereas the cuprates are classified as charge transfer insulators [9], where the doped holes are on the O sites. In the nickelates, there is hence a possibility of high-spin (S = 1) states forming on the doped Ni sites as a result of Hund c...

Nonreciprocal charge transport phenomena are studied theoretically for two-dimensional noncentrosymmetric superconductors under an external magnetic field B. Rashba superconductors, surface superconductivity on the surface of three-dimensional topological insulators (TIs), and transition metal dichalcogenides (TMD) such as MoS2 are representative systems, and the current-voltage I-V characteristics, i.e., V = V (I), for each of them is analyzed. V (I) can be expanded with respect to the current I as V (I) = j=1,∞ aj(B, T )I j , and the (B, T )-dependence of aj depends on the mechanism of the charge transport. Note that the magnetochiral anisotropy in the normal state is expressed by a1(B, T ) = R0 and a2(B, T ) = R0γB with the constant γ. Our analysis is based on the time-dependent Ginzburg-Landau (TDGL) theory, which contains up to third order terms in the momentum of the order parameter. Above the mean field superconducting transition temperature T0, the fluctuation of the superconducting order parameter gives the additional conductivity, i.e., paraconductivity. Extending the analysis of paraconductivity to the nonlinear response, we obtain the nonreciprocal charge transport expressed by a2(B, T ) = a1(T )γ(T )B, where γ converges to a finite value at T = T0. Below T0, the motion of vortices is relevant to the voltage drop, and the dependence of a1 and a2 on B and T is different depending on the system and mechanisms. For Rashba superconductors and superconducting surface state of three dimensional TIs under the in-plane magnetic field, the Kosterlitz-Thouless (KT) transition occurs at TKT, below which the vortices and anti-vortices are bound and the resistivity becomes zero. Therefore, a1(B, T ) and a2(B, T ) are defined only above TKT. In this case, the vortex contributions to the transport coefficients are a1(B, T ) = const. and a2(B, T ) ∼ B(T0 − T ) −1 for T → T0. It is also found near TKT that a1(B, T ) ∼ exp[−(bτc/τ ) 1/2 ] and a2(B, T ) ∼ Bτ −3/2 exp[−(bτc/τ ) 1/2 ] with the reduced temperatures τ (T ) = (T − TKT)/TKT, τc = τ (T0), and an order of unity constant b. Below TKT, both a1(B, T ) and a2(B, T ) vanish, and a3(B, T ) ∼ const. and a4(B, T ) ∼ B become the leading two terms in the expansion. On the other hand, for TMD with the magnetic field perpendicular to the 2D system, the KT transition is gone and the system remains resistive even well below TKT. In this case, there are two possible mechanisms for the nonreciprocal charge transport. One is the anisotropy of the damping constant for the motion of the vortex induced by the external magnetic field and current. In this case, a1(B, T ) ∼ B and a2(B, T ) ∼ B 2 . The other one is the ratchet potential acting on the vortex motion, which gives a1(B, T ) ∼ B and a2(B, T ) ∼ B. Based on these results, we propose the experiments to identify the mechanism of the nonreciprocal transport with the realistic estimates for the order of magnitude of the coefficients a1(B, T ) and a2(B, T ) for each case.

We study the prototype 5d pyrochlore iridate Y2Ir2O7 from first principles using the local density approximation and dynamical mean-field theory (LDA+DMFT). We map out the phase diagram in the space of temperature, onsite Coulomb repulsion U , and filling. Consistent with experiments, we find that an all-in-all-out ordered insulating phase is stable for realistic values of U . The trigonal crystal field enhances the hybridization between the j eff =1/2 and j eff = 3/2 states, and strong inter-band correlations are induced by the Coulomb interaction, which indicates that a three-band description is important. We demonstrate a substantial band narrowing in the paramagnetic metallic phase and non-Fermi liquid behavior in the electron/hole doped system originating from long-lived quasi-spin moments induced by nearly flat bands.PACS numbers: 71.15. Mb,71.27.+a,71.30.+h The competition and cooperation between spin-orbit coupling (SOC) and electron correlations induces novel phenomena in 4d and 5d transition metal oxides such as spin-orbit-assisted Mott insulators, topological phases, and spin liquids [1]. The pyrochlore iridates A 2 Ir 2 O 7 (A=Pr, Nd, Y, etc.) are an ideal system to study these phenomena because their magnetic and electronic states can be tuned by chemical substitution, pressure, and temperature (T ). Furthermore, intriguing phenomena such as correlated topological phases have been theoretically predicted on their geometrically frustrated crystal structure [1].In 2001, it was reported that these compounds show a crossover from metal to insulator with decreasing A 3+ ionic radii at high T [2] and a magnetic anomaly was found at low T for small A 3+ ionic radii [3]. For the metallic compound A=Pr, experiments revealed spinliquid behavior [4,5] and an unconventional anomalous Hall effect [6]. On the other hand, the Ir magnetic ordering has not been determined for a decade due to the strong neutron absorption by Ir and large magnetic contributions from rare-earth f moments on A 3+ . The magnetic order has only recently been identified as a noncollinear all-in-all-out order [see Fig. 1(a)Among the insulating compounds, Y 2 Ir 2 O 7 has the highest magnetic transition temperature and no f moments. This makes this compound a prototype system for studying strong electron correlations among 5d electrons. A pioneering local density approximation (LDA)+U study for this compound showed that the allin-all-out order is indeed stable at large on-site repulsion U [10]. It also proposed a topological Weyl semimetal as the ground state of some compounds in this series. This stimulated further theoretical studies on the topological nature and unconventional quantum criticality of 5d electrons on the pyrochlore lattice [1,[11][12][13][14][15][16][17][18][19].In pyrochlore iridates, the Ir atoms form a frustrated pyrochlore lattice, a corner-sharing network of tetrahedra [see Fig. 1(a) and Ref. 20]. The so-called j eff =1/2 picture was originally proposed for the insulating quasi-2D compound Sr 2 IrO 4 with the same e...

We study the symmetry-broken phases in two-and three-orbital Hubbard models with lifted orbital degeneracy using dynamical mean-field theory. On the technical level, we explain how symmetry relations can be exploited to measure the four-point correlation functions needed for the calculation of the lattice susceptibilities. In the half-filled two-orbital model with crystal-field splitting, we find an instability of the metallic phase to spin-orbital order with neither spin nor orbital moment. This ordered phase is shown to be related to the recently discovered fluctuating-moment induced spin-triplet superconducting state in the orbitally degenerate model with shifted chemical potential. In the three-orbital case, we consider the effect of a crystal-field splitting on the spin-triplet superconducting state in the model with positive Hund coupling, and the spin-singlet superconducting state in the case of negative Hund coupling. It is demonstrated that for certain crystal-field splittings the higher energy orbitals instead of the lower ones are relevant for superconductivity, and that T c can be slightly enhanced by the crystal-field effect. We comment on the implications of our results for the superconductivity in strontium ruthenates, and for the recently reported light-enhanced superconducting state in alkali-metal-doped fullerides.