2,868 research outputs found
Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors
The build-up of exciton populations in resonantly laser excited
semiconductors is studied microscopically. For excitation at the -exciton
resonance, it is shown that polarization with a strict -type radial symmetry
can be efficiently converted into an incoherent -type population. As a
consequence, inversion between the and exciton states can be obtained
leading to the appearance of significant terahertz gain.Comment: 4 pages, 3 figure
Neutral low-dimensional assemblies of a Mn(III) schiff base complex and octacyanotungstate(V) : synthesis, characterization, and magnetic properties
International audienceTwo novel low-dimensional molecular magnetic materials were prepared by the self-assembly of 3d- and 5d-metal complexes. These are the first neutral heterobimetallic cyanobridged compounds involving one anisotropic Mn(III) Schiff base complex and one octacyanotungstate(V) per molecular unit. A slow diffusion of the constituents’ solutions leads to the formation of the 0D crystalline complex 1, due to coordination of a water molecule to the Mn center, which prevents polymer formation. A rapid mixing of reagents results in the precipitation of the microcrystalline powder of complex 2, which based on the totality of experimental data, possesses a 1D polymeric structure. The magnetic studies have shown that antiferromagnetic exchange interactions prevail in 1 (J/kB = −13.1(7) K, D = −3.0(1.3) K, zJ' = −0.16(20) K and gav = 2.00(1)); while the presence of the significant intramolecular Mn(III)–W(V) ferromagnetic coupling through cyanide bridge is characteristic for 2 (J/kB = 46.1(5) K, gMn = 2.11(3), fixed gW = 2.0). Due to the weak interchain interactions, zJ′/kB = −0.8(2) K, and compound 2 is a metamagnet with the Néel temperature of 9.5 K undergoing a spin-flip transition at 2 kOe. The slow magnetization dynamics of 2 were investigated at a DC field of 0 and 2 kOe, giving the values of τ0 32(15) and 36(15) ps, respectively, well within the range typical for single-chain magnets (SCMs). The respective ∆τ/kB values were 48.4(1.2) and 44.9(1.0) K
Observation of Coulomb-Assisted Dipole-Forbidden Intraexciton Transitions in Semiconductors
We use terahertz pulses to induce resonant transitions between the
eigenstates of optically generated exciton populations in a high-quality
semiconductor quantum-well sample. Monitoring the excitonic photoluminescence,
we observe transient quenching of the exciton emission, which we attribute
to the terahertz-induced -to- excitation. Simultaneously, a pronounced
enhancement of the -exciton emission is observed, despite the -to-
transition being dipole forbidden. A microscopic many-body theory explains the
experimental observations as a Coulomb-scattering mixing of the 2 and 2
states, yielding an effective terahertz transition between the 1 and 2
populations.Comment: 5 pages, 3 figure
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