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We report measurements of the polar Kerr effect, proportional to the out-of-plane component of the magnetization, in thin films of the magnetically doped topological insulator (Cr0.12Bi0.26Sb0.62)2Te3. Measurements of the complex Kerr angle TK were performed as a function of photon energy in the range 0.8eV< ħω<3.0eV. We observed a peak in the real part of ΘK(ω) and zero crossing in the imaginary part that we attribute to a resonant interaction with a spin-orbit avoided crossing located ≈1.6 eV above the Fermi energy. The resonant enhancement allows measurement of the temperature and magnetic field dependence of ΘK in the ultrathin film limit, d≥2 quintuple layers (QL). We find a sharp transition to zero remanent magnetization at 6 K for d<8 QL, consistent with theories of the dependence of impurity spin interactions on film thickness and their location relative to topological insulator surfaces. [ABSTRACT FROM AUTHOR]

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate γ-Li2IrO3 (LIO), as well as the layered honeycomb iridate Na2IrO3 (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition, we measured the photoinduced changed in reflectance ΔR, as a function of time t, temperature T, and probe field polarization in both LIO and NIO. In LIO, ΔR(t,T) is anisotropic and comprises three T-dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, ΔR(t,T) has a single T-dependent component that is strikingly similar to the electronic excitation component of ΔR in LIO. Through analysis and comparison of ΔR(t,T) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order. [ABSTRACT FROM AUTHOR]

We report the observation of coherent oscillations associated with charge-density-wave (CDW) order in the underdoped cuprate superconductor YBa2Cu3O6+x by time-resolved optical reflectivity. Oscillations with frequency 1.87 THz onset at approximately 105 and 130 K for dopings of x = 0.67 (ortho-VIII) and x = 0.75 (ortho-III). respectively. Upon cooling below the superconducting critical temperature (Tc), the oscillation amplitude is enhanced, the phase shifts by π, and the frequency softens by δv/v ≈ 7%. A biquadratically coupled Landau-Ginzburg model qualitatively describes this behavior as arising from competition between superconducting and CDW orders. [ABSTRACT FROM AUTHOR]

We use pump-probe spectroscopy to measure the photoinduced reflectivity ΔR of the electron-doped cuprate superconductor Nd2-xCexCuO4+δ at a value of x near optimal doping, as a function of time, temperature, and laser fluence. We observe the onset of a negative ΔR signal at T" ≈ 75 K, above the superconducting transition temperature, Tc, of 23 K. The relatively slow decay of ΔR, compared to the analogous signal in hole doped compounds, allows us to resolve time-temperature scaling consistent with critical fluctuations. A positive ΔR signal onsets at Tc that we associate with superconducting order. We find that the two signals are strongly coupled below Tc, in a manner that suggests a repulsive interaction between superconductivity and another fluctuating order. [ABSTRACT FROM AUTHOR]

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe0.8Co0.2Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets. [ABSTRACT FROM AUTHOR]

We use pump-probe spectroscopy to measure the photoinduced reflectivity ΔR of the electron-doped cuprate superconductor Nd(2-x)Ce(x)CuO(4+δ) at a value of x near optimal doping, as a function of time, temperature, and laser fluence. We observe the onset of a negative ΔR signal at T(*)≈75 K, above the superconducting transition temperature, T(c), of 23 K. The relatively slow decay of ΔR, compared to the analogous signal in hole doped compounds, allows us to resolve time-temperature scaling consistent with critical fluctuations. A positive ΔR signal onsets at T(c) that we associate with superconducting order. We find that the two signals are strongly coupled below T(c), in a manner that suggests a repulsive interaction between superconductivity and another fluctuating order.