Spontaneous emission in inertial and dissipative nematic liquid crystals: the role of critical phenomena.

We develop a rigorous, field-theoretical approach to the study of spontaneous emission in inertial and dissipative nematic liquid crystals (LCs), disclosing an alternative application of the massive Stückelberg gauge theory to describe critical phenomena in these systems. This approach allows one not only to unveil the role of phase transitions in the spontaneous emission in LCs but also to make quantitative predictions for quantum emission in realistic nematics of current scientific and technological interest in the field of metamaterials. Specifically, we predict that one can switch on and off quantum emission in LCs by varying the temperature in the vicinities of the crystalline-to-nematic phase transition, for both the inertial and dissipative cases. We also predict from first principles the value of the critical exponent that characterizes such a transition, which we show not only to be independent of the inertial or dissipative dynamics, but also to be in good agreement with experiments. We determine the orientation of the dipole moment of the emitter relative to the nematic director that inhibits spontaneous emission, paving the way to achieve directionality of the emitted radiation, a result that could be applied in tuneable photonic devices such as metasurfaces and tuneable light sources.

Manifestation of hopping conductivity and granularity within phase diagrams of LaO1-x F x BiS2, Sr1-x La x FBiS2 and related BiS2-based compounds.

Layered BiS [Formula: see text] -based series, such as LaO [Formula: see text] F [Formula: see text] BiS [Formula: see text] and Sr [Formula: see text] La [Formula: see text] FBiS [Formula: see text] , offer ideal examples for studying normal and superconducting phase diagram of a solid solution that evolves from a nonmagnetic band-insulator parent. We constructed typical [Formula: see text] phase diagrams of these systems based on events occurring in thermal evolution of their electrical resistivity, [Formula: see text]. Overall evolution of these diagrams can be rationalized in terms of (i) Mott-Efros-Shklovskii scenario which, within the semiconducting [Formula: see text] regime ([Formula: see text] metal-insulator transition), describes the doping influence on the thermally activated hopping conductivity. (ii) A granular metal (superconductor) scenario which, within [Formula: see text], describes the evolution of normal and superconducting properties in terms of conductance g, Coulomb charging energy E c and Josephson coupling J; their joint influence is usually captured within a [Formula: see text] phase diagram. Based on analysis of the granular character of [Formula: see text], we converted the [Formula: see text] diagrams into projected g - T diagrams which, being fundamental, allow a better understanding of evolution of various granular-related properties (in particular the hallmarks of normal-state [Formula: see text] feature and superconductor-insulator transition) and how such properties are influenced by x, pressure or heat treatment.

d-f hybridization and quantum criticality in weakly-itinerant ferromagnets.

We investigate the unusual magnetic, thermodynamic and transport properties of nearly-critical, weakly-itinerant ferromagnets with the general formula UTX, where T=Rh, Co and X=Ge, Si. As a unique feature of these systems, we show how changes in the V(df) hybridization, which controls their proximity to a ferromagnetic instability, determine the evolution of the ground state magnetization, M(0), the Curie temperature, T(C), the density of states at the Fermi level, N(E(F)), the T(2) resistivity coefficient, A, and the specific heat coefficient, γ. The universal aspect of our findings comes from the dependence on only two parameters: the transition metal T(d) bandwidth, W(d), and the distance between the T(d) and U(f) band centers, C(T(d)) - C(U(f)). We discuss our results in connection to data for URh(1-x)Co(x)Ge.

Anisotropy of acceptor states in lightly doped cuprate superconductors.

We investigate the role of short wavelength antiferromagnetic correlations in the spatial structure of acceptor states doped into a two-dimensional Mott-Hubbard antiferromagnetic insulator. We extend the traditional effective mass approximation, by using the Green's function formalism and its spectral representation, and we show that when the electronic scattering at the ordering wavevector Q = (π,π) is strong enough to produce a momentum dependent scattering rate, Γ(k), the corresponding acceptor state envelope wavefunction becomes spatially anisotropic. Finally, we discuss the connection between our results and photoemission spectra in lightly doped La(2-x)Sr(x)CuO(4).

Skyrmions in a doped antiferromagnet.

Magnetization and magnetoresistance have been measured in insulating antiferromagnetic La2Cu0.97Li0.03O4 over a wide range of temperatures, magnetic fields, and field orientations. The magnetoresistance step associated with a weak ferromagnetic transition exhibits a striking nonmonotonic temperature dependence, consistent with the presence of Skyrmions.

Anisotropies in the optical ac and dc conductivities in lightly doped La(2 - x)Sr(x)CuO4: the role of deep and shallow acceptor states.

We investigate the origin of the optical ac and dc conductivity anisotropies observed in the low temperature orthorhombic phase of lightly doped, untwinned La(2 - x)Sr(x)NiO(4) single crystals. We show that these anisotropies can be naturally ascribed to the emergence of two odd parity, rotational-symmetry-broken, localized impurity acceptor states, one deeper and one shallower, resulting from the trapping of doped holes by the Coulomb potential provided by the Sr ions. These two lowest-energy, p-wave-like states are split by orthorhombicity and are partially filled with holes. This leaves a unique imprint in the optical ac conductivity, which shows two distinct far-infrared continuum absorption energies corresponding to the photoionization of the deep and shallow acceptor states. Furthermore, we argue that the existence of two independent and orthogonal channels for hopping conductivity, directly associated with the two orthorhombic directions, also quantitatively explains the observed low temperature anisotropies in the dc conductivity.

Magnetic and quasiparticle excitation spectra of an itinerant J1-J2 model for iron-pnictide superconductors.

We calculate the magnetic and quasiparticle excitation spectra of an itinerant J(1)-J(2) model for iron-pnictide superconductors. In addition to an acoustic spin-wave branch, the magnetic spectrum has a second, optical branch, resulting from the coupled four-sublattice magnetic structure. The spin-wave velocity has also a planar directional anisotropy, due to the collinear or striped antiferromagnetism. Within the magnetically ordered phase, the quasiparticle spectrum is composed of two Dirac cones, resulting from the folding of the magnetic Brillouin zone. We discuss the relevance of our findings to the understanding of both neutron scattering and photoemission spectroscopy results for SrFe(2)As(2).

Lightly doped La2-xSrxCuO4 as a Lifshitz helimagnet.

We study the static magnetic correlations in lightly doped La2-xSrxCuO4 within the framework of a dipolar frustration model for a canted antiferromagnet. We show that the stability of the canted Néel state for x < 2% is due to the Dzyaloshinskii-Moriya and XY anisotropies. For higher doping, the ground state is unstable towards a helicoidal magnetic phase, where the transverse components of the staggered magnetization rotate in a plane perpendicular to the orthorhombic b axis. Our theory reconciles, for the first time, the incommensurate peaks observed in elastic neutron scattering with Raman and magnetic susceptibility experiments in La2-xSrxCuO4 .

Interplay between disorder and quantum and thermal fluctuations in ferromagnetic alloys: the case of UCu2Si2-xGex.

We consider, theoretically and experimentally, the effects of structural disorder, quantum fluctuations, and thermal fluctuations in the magnetic and transport properties of certain ferromagnetic alloys. We study the particular case of UCu2Si2-xGex. The low temperature resistivity, rho(T,x), exhibits Fermi liquid behavior as a function of temperature T for all values of x, which can be interpreted as a result of the magnetic scattering of the conduction electrons from the localized U spins. The residual resistivity, rho(0,x), follows the behavior of a disordered binary alloy. The observed nonmonotonic dependence of the Curie temperature, T(c)(x), with x can be explained within a model of localized spins interacting with an electronic bath. Our results clearly show that the Curie temperature of certain alloys can be enhanced due to the interplay between quantum and thermal fluctuations with disorder.