Stimulated terahertz stokes emission of silicon crystals doped with antimony donors

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon (approximately 2.92 THz). The emission frequency covers the range of 4.6-5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation approximately 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center.     

Metallic normal state of Y1Ba2Cu3O(7-delta)

Flux flow was studied over an entire temperature range down to T approximately 2% of T(c) by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows rho approximately rho(n)B/H(c2), with a prefactor of order unity. Contrary to some speculation, rho(n) saturates to a finite residual value as T-->0, indicating a metallic (rho-->finite) rather than insulating (rho-->infinity) normal state, and the vortex dissipation continues to be conventional as T-->0.     

Crossed products by finite group actions with the Rokhlin property

We prove that a number of classes of separable unital C*-algebras are closed under crossed products by finite group actions with the Rokhlin property, including: (a) AI algebras, AT algebras, and related classes characterized by direct limit decompositions using semiprojective building blocks. (b) Simple unital AH algebras with slow dimension growth and real rank zero. (c) C*-algebras with real rank zero or stable rank one. (d) Simple C*-algebras for which the order on projections is determined by traces. (e) C*-algebras whose quotients all satisfy the Universal Coefficient Theorem. (f) C*-algebras with a unique tracial state. Along the way, we give a systematic treatment of the derivation of direct limit decompositions from local approximation conditions by homomorphic images which are not necessarily injective.     

The heat capacities of arsenic,phosphorous and GexSe1−x glasses below 1 K

The heat capacities of amorphous arsenic and phosphorous have been measured down to 0.1 K. Both glasses have heat capacities larger than the values calculated from the sound velocities, but in a-As the magnitude of the excess is significantly less than in vitreous silica. Ge_xSe_1?x glasses also show an excess heat capacity below 1 K, but in contrast to the behavior at higher temperatures the results do not vary systematically with composition.     

Three-valence-particle fission product 51 135Sb84

By analysis of fission product γ-ray data measured at Eurogam II using a ₂₄₈Cm source, yrast levels up to about 2 MeV in the N=84 three-particle nucleus ₁₃₅Sb have been identified. These levels are interpreted as π g^7/2ν f^7/2 ₂ and π g^7/2ν f^7/2h^9/2 states with the help of shell model calculations using empirical nucleon-nucleon interactions. Received: 30 July 1998     

Simplex s = ±i excitations in 141Xe

Excited levels in ¹⁴¹Xe, populated in spontaneous fission of ²⁴⁸Cm, were studied by means of prompt -ray spectroscopy, using the EUROGAMM2 array. Level scheme of ¹⁴¹Xe obtained in this work shows patterns characteristic of simplex symmetry with s=+i and s=-i bands present but low value of D₀ moment indicates that octupole correlations in Xe isotopes are systematically lower than in Ba nuclei. Received: 9 March 2000 / Accepted: 4 May 2000     

Laser-induced quantum coherence in a semiconductor quantum well

The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon resonant with the 1-3 intersubband transition, produces a pronounced narrow transparency feature in the 1-2 absorption line. This result can be understood in terms of all three states being simultaneously driven into "phase-locked" quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory.     

Properties of N=84, even-even nuclei populated in the spontaneous fission of 248Cm

Excited states in the neutron-rich, N=84 nuclei ¹³⁴Sn, ¹³⁶Te and ¹³⁸Xe, populated in the spontaneous fission of ²⁴⁸Cm, were studied to medium spins using the EUROGAM2 array. OXBASH code calculations support the experimental identification of maximum aligned configurations in these isotopes. Empirical shell model calculations agree with the proposed excitation energy of the neutron h_9/2 excitation in the ¹³²Sn region. A discrepancy between the observed and calculated excitation energy of the I^π= 12⁺ level in ¹³⁶Te indicates possible admixtures of collective excitations in this nucleus. Clear signs of collective excitations are observed in ¹³⁸Xe. Received: 10 November 1999 / Revised version: 22 December 1999     

Slow algebraic relaxation in quartic potentials and related results

We present a detailed report [see S. Sen et al., Phys. Rev. Lett. 77, 4855 (1996)] of our numerical and analytical studies on the relaxation of a classical particle in the potentials V(x)= +/-x(2)/2+x(4)/4. Both of the approaches confirm that at all temperatures, the relaxation functions (e.g., velocity relaxation function and position relaxation function) decay asymptotically in time t as sin(omega(0)t)/t. Numerically calculated power spectra of the relaxation functions show a gradual transition with increasing temperature from a single sharp peak located at the harmonic frequency omega(0) to a broad continuous band. The 1/t relaxation is also found when V(x) is a polynomial in powers of x(2) with a nonvanishing coefficient accompanying the x(4) term in V(x). Numerical calculations show that in the cases in which the leading term in V(x) behaves as x(2n) with integer n, the asymptotic relaxation exhibits 1/t(phi) decay where phi =1/(n-1). We briefly discuss the analytical approaches to relaxation studies in these strongly anharmonic systems using direct solution of the equation of motion and using the continued fraction formalism approach for relaxation studies. We show that the study of the dynamics of strongly anharmonic oscillators poses unique difficulties when studied via the continued fraction or any other time-series construction based approaches. We close with comments on the physical processes in which the insights presented in this work may be applicable.     

Hippocampal volume is related to cognitive decline and fornicial diffusion measures in multiple sclerosis

Purpose

To assess for associations between hippocampal atrophy and measures of cognitive function, hippocampal magnetization transfer ratio (MTR), and diffusion measures of the fornix, the largest efferent white matter tract from the hippocampus, in patients with multiple sclerosis (MS) and controls.

Materials and Methods

A total of 53 patients with MS and 20 age- and sex-matched healthy controls participated in cognitive testing and scanning including high spatial-resolution diffusion imaging and a T1-MPRAGE scan. Hippocampal volume and fornicial thickness measures were calculated and compared to mean values of fornicial transverse diffusivity, mean diffusivity, longitudinal diffusivity, fractional anisotropy, mean hippocampal MTR, and scores on measures of episodic memory, processing speed, and working memory tasks.

Results

In patients with MS, hippocampal volume was significantly related to fornicial diffusion measures (P < 7 × 10^− 4) and to measures of verbal (P = 0.030) and visual spatial (P = 0.004) episodic memory and a measure of information processing speed (P < 0.037).

Discussion

These results highlight the role of the hippocampus in cognitive dysfunction in patients with MS and suggest that measures of hippocampal atrophy could be used to capture aspects of disease progression.