Coherent THz synchrotron radiation from a storage ring with high-frequency RF system

The generation of brilliant, stable, and broadband coherent synchrotron radiation (CSR) in electron storage rings depends strongly on ring rf system properties such as frequency and gap voltage. We have observed intense coherent radiation at frequencies approaching the THz regime produced by the MIT-Bates South Hall Ring, which employs a high-frequency S-band rf system. The measured CSR spectral intensity enhancement with 2 mA stored current was up to 10,000 times above background for wave numbers near 3 cm(-1). The measurements also uncovered strong beam instabilities that must be suppressed if such a very high rf frequency electron storage ring is to become a viable coherent THz source.     

A characterization of plasma fluctuations within a Hall discharge

Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods     

Optical model of transient light scattering in ferroelectric liquid crystals

A static optical model is developed for the effect of field-induced transient scattering on coherent light transmission through ferroelectric liquid crystals. Scattering processes are described by introducing an optically anisotropic medium containing scatterers (transient domains). The results presented in the paper are obtained for a plane parallel layer of ferroelectric liquid crystals with a planar helicoidal structure under normal illumination with a linearly polarized plane wave. An analysis is presented of the coherent transmittance of the layer in static applied electric fields.     

Coherent photogalvanic valley Hall effect

The theory of the coherent photogalvanic valley Hall effect in two-dimensional systems with the Dirac spectrum of charge carriers is formulated. The study deals with a two-dimensional sample irradiated by two electromagnetic waves, at the fundamental and doubled frequencies. Both frequencies exceed the band gap of the material, whereas the wave with the fundamental frequency having circular polarization and a high intensity is taken into account in a nonperturbative manner. The wave at the doubled frequency is linearly polarized and the electrical conductivity of the two-dimensional system is calculated with respect to it. The effect under study manifests itself as the dc Hall current in the direction orthogonal to the electric field of the weak electromagnetic wave. It is assumed that, in equilibrium, the sample is in the insulating state with the completely occupied valence band and empty conduction band. The strong electromagnetic wave induces a nonequilibrium filling of the bands and the system passes to a strongly nonequilibrium steady state. The behavior of the Hall current in the case of nonequilibrium distribution functions is analyzed both including and disregarding the intraband relaxation and interband recombination.     

Bilayer coherent and quantum Hall phases: duality and quantum disorder

We consider a fully spin-polarized quantum Hall system with no interlayer tunneling at total filling factor nu = 1/k (where k is an odd integer) using the Chern-Simons-Ginzburg-Landau theory. Exploiting particle-vortex duality and the concept of quantum disordering, we find a large number of possible compressible and incompressible ground states, some of which may have relevance to recent experiments of Spielman et al. [Phys. Rev. Lett. 84, 5808 (2000)]. We find interlayer coherent compressible states without Hall quantization and interlayer incoherent incompressible states with Hall quantization in addition to the usual (k,k,k) Halperin states, which are both interlayer coherent and incompressible.     

Bridge between Abelian and non-Abelian fractional quantum Hall states

We propose a scheme to construct the most prominent Abelian and non-Abelian fractional quantum Hall states from K-component Halperin wave functions. In order to account for a one-component quantum Hall system, these SU(K) colors are distributed over all particles by an appropriate symmetrization. Numerical calculations corroborate the picture that K-component Halperin wave functions may be a common basis for both Abelian and non-Abelian trial wave functions in the study of one-component quantum Hall systems.     

Anyons and the quantum Hall effect—A pedagogical review

The dichotomy between fermions and bosons is at the root of many physical phenomena, from metallic conduction of electricity to super-fluidity, and from the periodic table to coherent propagation of light. The dichotomy originates from the symmetry of the quantum mechanical wave function to the interchange of two identical particles. In systems that are confined to two spatial dimensions particles that are neither fermions nor bosons, coined “anyons”, may exist. The fractional quantum Hall effect offers an experimental system where this possibility is realized. In this paper we present the concept of anyons, we explain why the observation of the fractional quantum Hall effect almost forces the notion of anyons upon us, and we review several possible ways for a direct observation of the physics of anyons. Furthermore, we devote a large part of the paper to non-abelian anyons, motivating their existence from the point of view of trial wave functions, giving a simple exposition of their relation to conformal field theories, and reviewing several proposals for their direct observation.     

Coherent optical phonons and parametrically coupled magnons induced by femtosecond laser excitation of the Gd(0001) surface

Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.     

Quantum coherence and W∞ × SU(2) algebra in bilayer quantum Hall systems

We analyze the bilayer quantum Hall (QH) system by mapping it to the monolayer QH system with spin degrees of freedom. By this mapping the tunneling interaction term is identified with the Zeeman term. We clarify the mechanism of a spontaneous development of quantum coherence based on the Chern-Simons gauge theory with the lowest-Landau-level projection taken into account. The symmetry group is found to be W_∞ × SU(2), which says that the spin rotation affects the total electron density nearby. Using it extensively we construct the Landau-Ginzburg theory of the coherent mode. Skyrmion excitations are topological solitions in this coherent mode. We point out that they are detectable by measuring the Hall current distribution.     

Possibility of new resonances in transient nonlinear spectroscopy

We predict the existence of new types of extra resonances in transient four wave mixing from a system with negligible collisions. These new resonances arise from the coherent pumping and disappear in the long time limit. We give explicit results for extra resonances in four wave mixing from several conventonally used systems.     

Spin pumping with coherent elastic waves

We show that the resonant coupling of phonons and magnons can be exploited to generate spin currents at room temperature. Surface acoustic wave pulses with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic waves in a ferromagnetic thin-film-normal-metal (Co/Pt) bilayer. We use the inverse spin Hall voltage in the Pt as a measure for the spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect.     

Ultrafast electronuclear dynamics of H(2) double ionization

The ultrafast electronic and nuclear dynamics of H(2) laser-induced double ionization is studied using a time-dependent wave packet approach that goes beyond the fixed nuclei approximation. The double ionization pathways are analyzed by following the evolution of the total wave function during and after the pulse. The rescattering of the first ionized electron produces a coherent superposition of excited molecular states which presents a pronounced transient H(+)H(-) character. This attosecond excitation is followed by field-induced double ionization and by the formation of short-lived autoionizing states which decay via double ionization. These two double ionization mechanisms may be identified by their signatures imprinted in the kinetic-energy distribution of the ejected protons.     

Precursor State to Unconventional Superconductivity in CeIrIn5

We present Hall effect and magnetoresistance measurements in the heavy fermion superconductor CeIrIn(5). At low temperature, a Kondo coherent state is established. Deviations from Kohler's rule and a quadratic temperature dependence of the cotangent of the Hall angle are reminiscent of properties observed in the high-temperature superconducting cuprates. A striking observation pertains to the presence of a precursor state--characterized by a change in the Hall mobility--that precedes the superconductivity in this material, in similarity to the pseudogap in the cuprate superconductors.     

Abelian and non-abelian Hall liquids and charge-density wave: quantum number fractionalization in one and two dimensions

Previously, we have demonstrated that, on a torus, the Abelian quantum Hall liquid is adiabatically connected to a charge-density wave as the smaller dimension of the torus is varied. In this work, we extend this result to the non-Abelian bosonic Hall state. The outcome of these works is the realization that the paradigms of quantum number fractionalization in one dimension (polyacetylene) and two dimensions (fractional quantum Hall effect) are, in fact, equivalent.     

Double point contact in quantum Hall line junctions

We show that multiple point contacts on a barrier separating two laterally coupled quantum Hall fluids induce Aharonov-Bohm (AB) oscillations in the tunneling conductance. These quantum coherence effects provide new evidence for the Luttinger liquid behavior of the edge states of quantum Hall fluids. For a two point contact, we identify coherent and incoherent regimes determined by the relative magnitude of their separation and the temperature. We analyze both regimes in the strong and weak tunneling amplitude limits as well as their temperature dependence. We find that the tunneling conductance should exhibit AB oscillations in the coherent regime, both at strong and weak tunneling amplitudes with the same period but with different functional form.     

超导体中库珀对的SU(2)与Glauber相干态——库珀对与约瑟夫森超流性的量子特性研究

本文利用类自旋算符证明BCS超导基态波函数是单个库珀对SU(2)相干态波函数的直积、且在一定条件下为库珀对体系的SU(2)相干态波函数。若两块处在BCS超导基态的超导体耦合在一起,则体系仍处在SU(2)相干态,且在一定条件下为定态超辐射态。在SU(2)群到谐振子群的收缩下,库珀对的SU(2)相干态变为Glauber相干态。讨论了两种情形下库珀对与约瑟夫森超流性的量子噪声、分布及二阶相关特性。     

Real-time observation of interference between atomic one-electron and two-electron excitations

We present results of real-time tracking of atomic two-electron dynamics in an autoionizing transient wave packet in krypton. A coherent superposition of two Fano resonances is excited with a femtosecond extreme-ultraviolet pulse. The evolution of the corresponding wave packet is subsequently probed with a delayed infrared pulse. In our specific case, we get access to the interference between one- and two-electron excitation channels in the launched wave packet, which is superimposed on its decay through autoionization. A simple model is able to account for the observed dynamical evolution of this wave packet.     

Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy

The noise characteristic of available laser sources limits the sensitivity of many types of nonlinear spectroscopy. We show how to maximize the sensitivity by optimizing the strength of a local oscillator wave in a heterodyne detection scheme without altering the amplitude of the wave being detected. The intensity profile of the optimum local oscillator closely matches that of the incident probe wave, but the optimum intensity is much less than that of the probe under realistic conditions. A general signal-to-noise analysis applicable to all nonlinear spectroscopy techniques is presented along with specific applications to coherent Raman spectroscopy, two-photon absorption, saturation spectroscopy, and optical coherent transient techniques. A simple optimization procedure employing polarization selection rules is described. Detailed calculations are performed for the case of TEM₀₀ waves interacting via a third-order nonlinear susceptibility and for the case where the sample is simultaneously probed at many different frequency combinations. Supported by the National Science Foundation and Office of Naval Research.