The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ-ðBEDTÀTTFÞ 2 Cu½NðCNÞ 2 Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T Ã ≃ 50 K, far higher than the equilibrium transition temperature T C ¼ 12.5 K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

Herein, we offer a simple but crucial case for rational design and syntheses of infrared nonlinear optical (NLO) materials by employing state-of-the-art AgGaS 2 as a parent model. On the basis of inheriting structural advantages of AgGaS 2 , an as-grown CuZnPS 4 crystal exhibits a sharply enlarged energy gap (3.0 eV) benefiting from the cosubstitution of Ag with lighter Cu/Zn. Remarkably, the valence electron distribution of CuZnPS 4 is optimized by bringing in cation vacancy defects, which significantly reinforce the second harmonic generation (SHG) (3 × AgGaS 2 ) and compensate for the adverse effect of band gap enlargement. Careful experimental and theoretical investigations illustrate that CuZnPS 4 strikes a desirable balance among a strong SHG response, good phase matchability, large band gap, high laser damage threshold, outstanding physicochemical stability, low melting point, and cost-effective but nontoxic composition, which might shed light on follow-up design and exploratory synthesis of NLO materials.

Coulomb repulsion among conduction electrons in solids hinders their motion and leads to a rise in resistivity. A regime of electronic phase separation is expected at the first-order phase transition between a correlated metal and a paramagnetic Mott insulator, but remains unexplored experimentally as well as theoretically nearby T = 0. We approach this issue by assessing the complex permittivity via dielectric spectroscopy, which provides vivid mapping of the Mott transition and deep insight into its microscopic nature. Our experiments utilizing both physical pressure and chemical substitution consistently reveal a strong enhancement of the quasi-static dielectric constant ε1 when correlations are tuned through the critical value. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory. Our findings suggest a similar ’dielectric catastrophe’ in many other correlated materials and explain previous observations that were assigned to multiferroicity or ferroelectricity.

Transport, magnetic and optical investigations on EuRbFe 4 As 4 single crystals evidence that the ferromagnetic ordering of the Eu 2+ magnetic moments at T N = 15 K, below the superconducting transition (Tc = 36 K), affects superconductivity in a weak but intriguing way. Upon cooling below T N , the zero resistance state is preserved and the superconductivity is affected by the in-plane ferromagnetism mainly at domain boundaries; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below Tc, associated with the opening of a complete superconducting gap at 2∆ = 10 meV. A gap smaller than the weak coupling limit suggests the strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.New members of the iron-pnictide family, the so-called 1144-compounds, attract interest recently because the alternating layers of alkaline A and alkaline-earth B cations produce two different kinds of As sites [1][2][3][4]. These materials can be viewed as the intergrowth of A-122 and B-122 iron-pnictides and they are naturally hole doped. The parent compounds are superconducting with transition temperatures T c around 35 K, higher than most of the 122materials; no spin-density-wave order has been observed. Among all possible candidates, Eu-based 1144-systems are even more intriguing, since the Eu-sublattice orders ferromagnetically below a critical temperature T N ≈ 15 K [5,6], similar to the 122-counterpart EuFe 2 As 2 [7-11]. Ferromagnetic order deep inside the superconducting state is very rare, in general [12,13]; hence the "ferromagnetic superconductor" EuRbFe 4 As 4 might pave the way towards realization of a "superconducting ferromagnet" [14-16]. However, the exact nature of the Eu magnetic order and its effect on superconductivity is unresolved [5, 6] because single crystals have been synthesized only recently.In this Letter we focus on the interplay between superconductivity and ferromagnetism in EuRbFe 4 As 4 single crystals. We report comprehensive investigations comprising transport, magnetic and optical measurements combined with microscopic studies of the vortex dynamics. The infrared spectra show a clear gap opening around 80 cm −1 below T c = 36 K that is slightly reduced compared to the value expected from the BCS theory. We relate this small value to the multiband character of superconductivity as well as to the depairing (orbital) effects of super-currents screening the ferromagnetic domains. A surprisingly weak effect on the superconducting condensate has been observed upon magnetic ordering indicating a rather weak interaction between Eu-and Fe-sublattices.Single crystals of EuRbFe 4 As 4 are obtained according to Ref. [4,5,17,18]; they exhibit shiny ab-faces of approximately 1 mm in size. The structure of the compound is presented in Fig. 1(a). The crystals are characterized by x-ray, electrical transport, and magnetic susceptibility measurements. In Fig. 1(a) we plot the dc ...

We measured the reflectivity of the multifold semimetal RhSi in a frequency range from 80 to 20 000 cm −1 (10 meV -2.5 eV) at temperatures down to 10 K. The optical conductivity, calculated from the reflectivity, is dominated by the free-carrier (Drude) contribution below 1000 cm −1 (120 meV) and by interband transitions at higher frequencies. The temperature-induced changes in the spectra are generally weak -only the Drude bands narrow upon cooling with an unscreened plasma frequency being constant with temperature at approximately 1.4 eV, in agreement with a weak temperature dependence of the free-carrier concentration determined by Hall measurements. The interband portion of conductivity exhibits two linear-in-frequency regions below 5000 cm −1 (∼ 600 meV), a broad flat maximum at around 6000 cm −1 (750 meV), and a further increase starting around 10 000 cm −1 (∼ 1.2 eV). We assign the linear behavior of the interband conductivity to transitions between the linear bands near the band crossing points. Our findings are in accord with the predictions for the low-energy conductivity behavior in multifold semimetals and with earlier computations based on band structure calculations for RhSi.

Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H 2 O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at a distance of~5 Å with an interchannel separation of~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T 0 ≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.

The superconductivity precursor phenomena in high temperature cuprate superconductors is studied by direct measurements of the superconducting condensate with the use of the c-axis optical conductivity of YBa2(Cu1-xZnx)3Oy for several doping levels (p) as well as for several Zn concentrations. Both the real and imaginary parts of the optical conductivity clearly show that the superconducting carriers persist up to the high temperatures Tp that is higher than the critical temperature Tc but lower than the pseudogap temperature T*. Tp increases with reducing doping level like T*, but decreases with Zn substitution unlike T*.

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