In the last two decades non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. From this perspective, multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. For example, the nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Furthermore, recent experimental...

Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach

In the last two decades, non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. Multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice, and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. The nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Recent experimental developments have made possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. 
Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases, even though the discussion will extend also to other topical systems. The necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontier in physics.

The magnetic properties of superconducting alloys. part i

We report the time-resolved excited state ultrafast dynamics of single unit cell (1 UC) thick FeSe films on SrTiO_{3} (STO), with FeTe capping layers. By measuring the photoexcited quasiparticles' density and lifetime, we unambiguously identify a superconducting (SC) phase transition, with a transition temperature T_{c} of 68 (-5/+2)  K and a SC gap of Δ(0)=20.2±1.5  meV. The obtained electron-phonon coupling strength λ is as large as 0.48, demonstrating the likely crucial role of electron-phonon coupling in the high temperature superconductivity of the 1 UC FeSe on STO systems. We further find a 0.05 THz coherent acoustic phonon branch in the capping layer, which provides an additional decay channel to the gluing bosons.

Ultrafast Dynamics Evidence of High Temperature Superconductivity in Single Unit Cell FeSe on SrTiO 3

The phase diagram for the LaBCCO solid solutions La 1.5- x Ba 1.5+ x-y Ca y Cu 3 O z has been determined in air at 950°C. Solid solution form over the complete composition range 0⩽ x ⩽0.5, 0⩽ y ⩽0.5 apart from composition at high x and low y , centred on the La-123 composition LaBa 2 Cu 3 O z . The LaBCCO solid solutions are tetragonal at all oxygen contents and are structurally distinct from tetragonal/orthorhombic La-123, which can be prepared as a stoichiometric phase by reaction in Ar. T c values of the LaBCCO solid solutions increase linearly with x at constant y and with y at constant x for samples prepared under the same conditions. Maximum T c is ∼78 K for x = y =0.5 for samples that were oxygen annealed at 400°C. The melting behaviour of the LaBCCO solid solutions has been determined. Melting temperatures increase with both decreasing x and y ; the La-336 composition has the highest melting temperature (∼1180°C) and melts incongruently to La 2 CuO 4 + liquid.

Superconducting La1.5−xBa1.5+x−yCayCu3Oz solid solutions I. Phase diagram, cation stoichiometry and Tc data

The temperature dependence of the local structure of liquid Sb has been studied by x-ray absorption spectroscopy. It is shown that about 10% of the atoms with coordination of 3 and weak Peierls distortion exist in liquid Sb just above its melting point. The Peierls distortion weakens gradually with increasing temperature and vanishes at about 750 degrees C. This structural variation in liquid Sb is different from the normal liquid-liquid phase transition. This work reveals the relationship between the variation in the local structure and the change in the physical properties, such as the electrical resisitvity of liquid Sb, with temperature. The complete agreement between the measured electrical resistivity values during heating and cooling processes suggests that the structural units with the features of a rhombohedron appear above the melting point of Sb during solidification. (c) 2008 American Institute of Physics.

Quasiparticle relaxation dynamics in n-type superconductor La2-xCexCuO4

The composition dependence of superconductivity and crystal structure in La(Ba1−xCax)2Cu3O7−y system was determined by the resistivity measurements and X-ray diffraction analysis. The superconducting transition temperature is raised with the increase of Ca content till x=0.6, at which the zero resistance temperature of the sample is 81.5 K. In the meanwhile, the crystal structure of the sample changed from tetragonal (x=0) to orthorhombic structure (x=0.2, 0.4, 0.6). With further increase of Ca content, the superconductivity decrease for the sample of x=0.8 with mixed phases including the orthorhombic oxygen-deficient perovskite-like (ODP) structure and no superconducting transition is found at 4.2 K for the sample of x=1 without the ODP structure. A possible explanation of these experimental results is given.

COMPOSITION DEPENDENCE OF SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE IN La(Ba1−xCax)2Cu3O7−y SYSTEM

The electric and magnetic measurements have been carried out on a polycrystalline single phase SmBa2Cu3O7 sample. The normal state susceptibility of it consists of a temperature-dependent Curie-Weiss paramagnetic term with an effective Bohr magneton number peff=1.1 and the Curie-Weiss temperature θ=38 K and temperature-independent terms: the Van Vleck and Pauli paramagnetism and the ionic core and Landau diagmagnetism. The Pauli susceptibility derived from the temperature-independent normal-state susceptibility of YBa2Cu3O7 is so large that it is due to the strong electron-electron interactions among conduction electrons. We deduce the low critical field Hc1(1.5 K), which is 580 Oe and the critical magnetization current density which is of the order of 104 A/cm2 at 5 kOe and decreases with increasing applied field from the magnetization hysteresis loop of SmBa2Cu3O7 for fields up to 60 kOe and temperature at 1.5 K. Some magnetic and superconductive parameters such as Hc(0), kGL, χ, λ, etc. are also given by using the values of the low and upper critical fields and the energy gap within the Ginzburg-Landau theory.

N. Cao

Papers

Quasiparticle relaxation dynamics in n-type superconductor La2-xCexCuO4

COMPOSITION DEPENDENCE OF SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE IN La(Ba1−xCax)2Cu3O7−y SYSTEM

Magnetic characteristics of normal and superconductive states in smba2cu3o7