Search Results (10)

Fluorenone derivatives represent promising candidates for electron-transport materials in organic electronic devices. Given that anionic species serve as electron-transfer carriers in electron-transport materials, it is highly desirable to isolate and characterize the radical anions and dianions of indenofluorened derivatives (IFO). In this work, the reduction of three indenofluorenedione derivatives (IFO, 1, 2 and 3) with potassium resulted in three radical anion salts (1K[(crypt-222)], 2K[(crypt-222)] and 3K) and one dianion salt (2[K(crypt-222)]₂). Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that 1K[(crypt-222)] and 2K[(crypt-222)] have a full delocalization of the unpaired electron which is supported by calculated spin density distributions. We demonstrate that the polarization of electron spin in 3K is induced by potassium ion coordination through single-crystal X-ray structure analysis and DFT calculations, suggesting the electrostatic effect by potassium ion has a significant influence on the spin density modulation. Superconducting quantum interference device (SQUID) measurements and DFT calculations show that 2[K(crypt-222)]₂ has an open-shell singlet base with a large singlet-triplet energy gap (ΔE_os-t = −7.40 kcal mol⁻¹) so that the excited triplet state is not accessible at room temperature. Full article

A wide class of materials with different crystal and electronic structures including quasi-2D unconventional superconductors, such as cuprates, nickelates, ferropnictides/chalcogenides, ruthenate Sr${}_2$RuO${}_4$, and 3D systems, such as manganites RMnO${}_3$, ferrates (CaSr)FeO${}_3$, nickelates RNiO${}_3$, silver oxide AgO, are based on Jahn–Teller $3d$ and $4d$ ions. These unusual materials, called Jahn–Teller (JT) magnets, are characterized by an extremely rich variety of phase states, spanning from non-magnetic and magnetic insulators to unusual metallic and superconducting states. The unconventional properties of JT magnets can be attributed to the instability of their highly symmetric Jahn–Teller “progenitors” with the ground orbital E-state with repect to charge transfer, anti-Jahn–Teller d-d disproportionation, and the formation of a system of effective local composite spin–singlet or spin–triplet, electronic, or hole S-type bosons moving in a non-magnetic or magnetic lattice. We consider specific features of the anti-JT-disproportionation reaction, properties of the electron–hole dimers, possible phase states and effective Hamiltonians for single- and two-band JT magnets, concluding with a short overview of physical properties for actual JT magnets. Full article

This review covers several recent developments in the physics of dense QCD with an emphasis on the impact of multiple phase transitions on astrophysical manifestations of compact stars. To motivate the multi-phase modeling of dense QCD and delineate the perspectives, we start with a discussion of the structure of its phase diagram and the arrangement of possible color-superconducting and other phases. It is conjectured that pair-correlated quark matter in $\beta$-equilibrium is within the same universality class as spin-imbalanced cold atoms and the isospin asymmetrical nucleonic matter. This then implies the emergence of phases with broken space symmetries and tri-critical (Lifshitz) points. The beyond-mean-field structure of the quark propagator and its non-trivial implications are discussed in the cases of two- and three-flavor quark matter within the Eliashberg theory, which takes into account the frequency dependence (retardation) of the gap function. We then construct an equation of state (EoS) that extends the two-phase EoS of dense quark matter within the constant speed of sound parameterization by adding a conformal fluid with a speed of sound $c_{\mathrm{conf}.}=1/\sqrt{3}$ at densities $\ge 10n_{\mathrm{sat}}$, where $n_{\mathrm{sat}}$ is the saturation density. With this input, we construct static, spherically symmetrical compact hybrid stars in the mass–radius diagram, recover such features as the twins and triplets, and show that the transition to conformal fluid leads to the spiraling-in of the tracks in this diagram. Stars on the spirals are classically unstable with respect to the radial oscillations but can be stabilized if the conversion timescale between quark and nucleonic phases at their interface is larger than the oscillation period. Finally, we review the impact of a transition from high-temperature gapped to low-temperature gapless two-flavor phase on the thermal evolution of hybrid stars. Full article

First-principles calculations for underdoped La_2−xSr_xCuO₄ (LSCO) have revealed a Fermi surface consisting of spin-triplet (KS) particles at the antinodal Fermi-pockets and spin-singlet (SS) particles at the nodal Fermi-arcs in the presence of AF local order. By performing a unique method of calculating the electronic-spin state of overdoped LSCO and by measurement of the spin-correlation length by neutron inelastic scattering, the origin of the phase-diagram, including the pseudogap phase in the high temperature superconductor, Sr-doped copper-oxide LSCO, has been elucidated. We have theoretically solved the long-term problem as to why the angle-resolved photoemission spectroscopy (ARPES) has not been able to observe Fermi pockets in the Fermi surface of LSCO. As a result, we show that the pseudogap region is bounded below the characteristic temperature T*(x) and above the superconducting transition temperature T_c(x) in the T vs. x phase diagram, where both the AF order and the KS particles in the Fermi pockets vanish at T*(x), whilst KS particles contribute to d-wave superconductivity below T_c. We also show that the relationship T*(x_c) = T_c(x_c) holds at x_c = 0.30, which is consistent with ARPES experiments. At T*(x), a phase transition occurs from the pseudogap phase to an unusual metallic phase in which only the SS particles exist. Full article

We report the finding of a novel pairing state in a newly discovered superconductor Na${}_2$Cr${}_3$As${}_3$. This material has a non-centrosymmetric quasi-one-dimensional crystal structure and is superconducting at $T_{\mathrm{C}}\sim$ 8.0 K. We find that the magnetic penetration depth data suggests the presence of a nodal line $p_z$-wave pairing state with zero magnetic moment using transverse-field muon-spin rotation (TF-$\mu$SR) measurements. The nodal gap observed in Na${}_2$Cr${}_3$As${}_3$ compound is consistent with that observed in isostructural (K,Cs)${}_2$Cr${}_3$As${}_3$ compounds using TF-$\mu$SR measurements. The observed pairing state is consistent with a three-band model spin-fluctuation calculation, which reveals the $S_z=0$ spin-triplet pairing state with the $sin{k}_z$ pairing symmetry. The long-sought search for chiral superconductivity with topological applications could be aided by such a novel triplet $S_z=0$p-wave pairing state. Full article

We theoretically investigated the proximity effect in SN${}_{SO}$F and SF’F structures consisting of a superconductor (S), a normal metal (N${}_{SO}$), and ferromagnetic (F’,F) thin films with spin–orbit interaction (SOI) in the N${}_{SO}$ layer. We show that a normal layer with spin–orbit interaction effectively suppresses triplet correlations generated in a ferromagnetic layer. Due to this effect, the critical temperature of the superconducting layer in the SN${}_{SO}$F multilayer turns out to be higher than in a similar multilayer without spin–orbit interaction in the N layer. Moreover, in the presence of a mixed type of spin–orbit interaction involving the Rashba and Dresselhaus components, the SN${}_{SO}$F structure is a spin valve, whose critical temperature is determined by the direction of the magnetization vector in the F layer. We calculated the control characteristics of the SN${}_{SO}$F spin valve and compared them with those available in traditional SF’F devices with two ferromagnetic layers. We concluded that SN${}_{SO}$F structures with one controlled F layer provide solid advantages over the broadly considered SF’F spin valves, paving the way for high-performance storage components for superconducting electronics. Full article

Starting with a minimal model for the CuO${}_2$ planes with the on-site Hilbert space reduced to only three effective valence centers [CuO${}_4$]${}^{7-,6-,5-}$ (nominally Cu${}^{1+,2+,3+}$) with different conventional spin and different orbital symmetry, we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory. Unconventional bosonic superconducting phase related with a two-particle quantum transport is shown to compete with antiferromagnetic insulating phase, charge order, and metallic Fermi liquid via phase separation regime. Full article

At a temperature of roughly 1 K, ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state has not even been understood after 25 years of study. More recently, it has been found that critical temperatures can be enhanced by the application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an “instability” approach and seek divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound, the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain, electrons residing on all orbitals contributing to the Fermiology become more coherent, while the electrons of the Ru-$d_{xy}$ character become heavier, and the electrons of the Ru-$d_{xz,yz}$ characters become lighter. In the process, Im $\chi (\mathbf{q},\omega )$ increases rapidly around $\mathbf{q} = (0.3,0.3,0)2\pi /a$ and $\mathbf{q} = (0.5,0.25,0)2\pi /a$, while it gets suppressed at all other commensurate vectors, in particular at $\mathbf{q} = 0$, which is essential for spin-triplet superconductivity. We observe that the magnetic anisotropy under strain drops smoothly, which is concomitant with the increment in singlet instability. Thus, the triplet superconducting instability remains the lagging instability of the system, and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. However, since this happens even without spin-orbit coupling, we believe it is primarily the enhancement in the spin fluctuation glue around quasi-anti-ferromagnetic vectors that drives the Cooper pairing instead of the magnetic anisotropy. At large strain, an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, ab initio description of the one-particle properties and two-particle susceptibilities, based on the quasiparticle self-consistent GW approximation augmented by dynamical mean field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties. Full article

Recent work done on the time reversal symmetry (TRS) breaking superconductors is reviewed in this paper. The special attention is paid to Sr ${}_2$ RuO ${}_4$ believed to be spin triplet chiral p-wave superconductor which break TRS and is expected to posses non-trivial topological properties. The family of TRS breaking superconductors is growing relatively fast, with many of its newly discovered members being non-centrosymmetric. However not only Sr ${}_2$ RuO ${}_4$ but also many other superconductors which possess center of inversion also break TRS. The TRS is often identified by means of the muon spin relaxation ( $\mu$ SR) and the Kerr effect. Both methods effectively measure the appearance of the spontaneous bulk magnetic field below superconducting transition temperature. This compound provides an example of the material whose many band, multi-condensate modeling has enjoyed a number of successes, but the full understanding has not been achieved yet. We discuss in some details the properties of the material. Among them is the Kerr effect and by understanding has resulted in the discovery of the novel mechanism of the phenomenon. The mechanism is universal and thus applicable to all systems with multi-orbital character of states at the Fermi energy. Full article

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr₂RuO₄, which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures T_c that are consistently as high as 1.4 K, as well as the properties of these crystals. Full article