Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors

We develop a theory describing the effects of many-particle Coulomb correlations on the coherent ultrafast nonlinear optical response of semiconductors and metals. Our approach is based on a mapping of the nonlinear optical response of the ``bare'' system onto the linear response of a ``dressed'' system. The latter is characterized by effective time-dependent optical transition matrix elements, electron/hole dispersions, and interaction potentials, which in undoped semiconductors are determined by the single-exciton and two-exciton Green functions in the absence of optical fields. This mapping is achieved by eliminating the optically-induced charge fluctuations from the Hamiltonian using a Van Vleck canonical transformation. It takes into account all many-body contributions up to a given order in the optical fields as well as important Coulomb-induced quantum dynamics to all orders in the optical field. Our approach allows us to distinguish between optical nonlinearities of different origins and provides a physically-intuitive interpretation of their manifestations in ultrafast coherent nonlinear optical spectroscopy.Comment: 24 page

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We present an absorption study of the neutral and positively charged exciton (trion) under the influence of a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the hole gas is created by the formation of trions. The evolution of these populations is studied, including the spin flip and trion formation processes. The contributions of several mechanisms to intensity changes are evaluated, including phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in p-doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding heavy holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blockedComment: 4 pages, 4 figure

Transverse sound in a magnetic field in UPt_3

We have propagated transverse sound in a magnetic field in the basal plane of UPt_3, with the polarization vector oriented both in the basal plane and perpendicular to it. We observe a strong anisotropy in the magnetic field dependence of the attenuation for the two polarizations. Using a simple phenomenological model, we can understand the low-temperature field dependence as a natural consequence of the anisotropy with temperature in zero field reported earlier [Phys. Rev. Lett. 56, 1078 (1986)]. However, for increasing temperatures there are significant deviations from this model. In no case do we find evidence for new superconducting phases in a magnetic field

Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells

Pump-probe experiments are used as a controllable way to investigate the properties of photoexcited semiconductors, in particular, the absorption saturation. We present an experiment-theory comparison for ZnSe quantum wells, investigating the energy renormalization and bleaching of the excitonic resonances. Experiments were performed with spin-selective excitation and above-bandgap pumping. The model, based on the semiconductor Bloch equations in the screened Hartree-Fock approximation, takes various scattering processes into account phenomenologically. Comparing numerical results with available experimental data, we explain the experimental results and find that the electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics, modeling of femtosecond pump-probe experiments, electron spin-flip tim

Pump Built-in Hamiltonian Method for Pump-Probe Spectroscopy

We propose a new method of calculating nonlinear optical responses of interacting electronic systems. In this method, the total Hamiltonian (system + system-pump interaction) is transformed into a different form that (apparently) does not have a system-pump interaction. The transformed Hamiltonian, which we call the pump built-in Hamiltonian, has parameters that depend on the strength of the pump beam. Using the pump built-in Hamiltonian, we can calculate nonlinear responses (responses to probe beams as a function of the pump beam) by applying the {\em linear} response theory. We demonstrate the basic idea of this new method by applying it to a one-dimensional, two-band model, in the case the pump excitation is virtual (coherent excitation). We find that the exponent of the Fermi edge singularity varies with the pump intensity.Comment: 6 page

Exotic superconductivity in the coexistent phase of antiferromagnetism and superconductivity in CeCu2(Si0.98Ge0.02)2: A Cu-NQR study under hydrostatic pressure

We report a pressure ($P$) effect on CeCu$_2$(Si$_{0.98}$Ge$_{0.02}$)$_2$ where an antiferromagnetic (AFM) order at $T_N \sim$ 0.75 K coexists with superconductivity below $T_c \sim$ 0.4 K\@. At pressures exceeding $P = 0.19$ GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at $P = 0$ GPa is found to evolve into a typical heavy-fermion one with a line-node gap above $P = 0.91$ GPa\@. We highlight that the anomalous enhancement in nuclear spin-lattice relaxation rate $1/T_1$ that follows a $T_1T$ = const. behavior well below $T_c$ at $P$ = 0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity.Comment: 5 pages with 4 figures embedded in the text. To be published in J. Phys. Soc. Jp

Polar type density of states in non-unitary odd-parity superconducting states of gap with point nodes

It is shown that the density of states (DOS) proportional to the excitation energy, the so-called polar like DOS, can arise in the odd-parity states with the superconducting gap vanishing at points even if the spin-orbit interaction for Cooper pairing is strong enough. Such gap stuructures are realized in the non-unitary states, F_{1u}(1,i,0), F_{1u}(1,varepsilon,varepsilon^{2}), and F_{2u}(1,i,0), classified by Volovik and Gorkov, Sov. Phys.-JETP Vol.61 (1985) 843. This is due to the fact that the gap vanishes in quadratic manner around the point on the Fermi surface. It is also shown that the region of quadratic energy dependence of DOS, in the state F_{2u}(1,varepsilon,varepsilon^{2}), is restricted in very small energy region making it difficult to distinguish from the polar-like DOS.Comment: 5 pages, 3 figures, submitted to J. Phys.: Condens. Matter Lette