The infrared conductivity of Nax_xCoO2_2: evidence of gapped states

We present infrared ab-plane conductivity data for the layered cobaltate Na$_x$CoO$_2$ at three different doping levels ($x=0.25, 0.50$, and 0.75). The Drude weight increases monotonically with hole doping, $1-x$. At the lowest hole doping level $x$=0.75 the system resembles the normal state of underdoped cuprate superconductors with a scattering rate that varies linearly with frequency and temperature and there is an onset of scattering by a bosonic mode at 600 \cm. Two higher hole doped samples ($x=0.50$ and 0.25) show two different-size gaps (110 \cm and 200 \cm, respectively) in the optical conductivities at low temperatures and become insulators. The spectral weights lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks at 200 \cm and 800 \cm, respectively. We propose that the two gapped states of the two higher hole doped samples ($x$=0.50 and 0.25) are pinned charge ordered states.Comment: 4 pages, 3 figure

Finite-size effect of antiferromagnetic transition and electronic structure in LiFePO4

The finite-size effect on the antiferromagnetic (AF) transition and electronic configuration of iron has been observed in LiFePO4. Determination of the scaling behavior of the AF transition temperature (TN) versus the particle-size dimension (L) in the critical regime 1-TN(L)/TN(XTL)\simL^-1 reveals that the activation nature of the AF ordering strongly depends on the surface energy. In addition, the effective magnetic moment that reflects the electronic configuration of iron in LiFePO4 is found to be sensitive to the particle size. An alternative structural view based on the polyatomic ion groups of (PO4)3- is proposed.Comment: To be published in Phys. Rev. B - Rapid Communicatio

Critical properties of the unconventional spin-Peierls system TiOBr

We have performed detailed x-ray scattering measurements on single crystals of the spin-Peierls compound TiOBr in order to study the critical properties of the transition between the incommensurate spin-Peierls state and the paramagnetic state at Tc2 ~ 48 K. We have determined a value of the critical exponent beta which is consistent with the conventional 3D universality classes, in contrast with earlier results reported for TiOBr and TiOCl. Using a simple power law fit function we demonstrate that the asymptotic critical regime in TiOBr is quite narrow, and obtain a value of beta_{asy} = 0.32 +/- 0.03 in the asymptotic limit. A power law fit function which includes the first order correction-to-scaling confluent singularity term can be used to account for data outside the asymptotic regime, yielding a more robust value of beta_{avg} = 0.39 +/- 0.05. We observe no evidence of commensurate fluctuations above Tc1 in TiOBr, unlike its isostructural sister compound TiOCl. In addition, we find that the incommensurate structure between Tc1 and Tc2 is shifted in Q-space relative to the commensurate structure below Tc1.Comment: 12 pages, 8 figures. Submitted to Physical Review

Temporal Dynamics of Photon Pairs Generated by an Atomic Ensemble

The time dependence of nonclassical correlations is investigated for two fields (1,2) generated by an ensemble of cold Cesium atoms via the protocol of Duan et al. [Nature Vol. 414, p. 413 (2001)]. The correlation function R(t1,t2) for the ratio of cross to auto-correlations for the (1,2) fields at times (t1,t2) is found to have a maximum value Rmax=292(+-)57, which significantly violates the Cauchy-Schwarz inequality R<=1 for classical fields. Decoherence of quantum correlations is observed over 175 ns, and is described by our model, as is a new scheme to mitigate this effect.Comment: 5 pages, 5 figure

Double-dot charge transport in Si single electron/hole transistors

We studied transport through ultra-small Si quantum dot transistors fabricated from silicon-on-insulator wafers. At high temperatures, 4K<T<100K, the devices show single-electron or single-hole transport through the lithographically defined dot. At T<4K, current through the devices is characterized by multidot transport. From the analysis of the transport in samples with double-dot characteristics, we conclude that extra dots are formed inside the thermally grown gate oxide which surrounds the lithographically defined dot.Comment: 4 pages, 5 figures, to appear in Appl. Phys. Let