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

The Sender-Excited Secret Key Agreement Model: Capacity, Reliability and Secrecy Exponents

We consider the secret key generation problem when sources are randomly excited by the sender and there is a noiseless public discussion channel. Our setting is thus similar to recent works on channels with action-dependent states where the channel state may be influenced by some of the parties involved. We derive single-letter expressions for the secret key capacity through a type of source emulation analysis. We also derive lower bounds on the achievable reliability and secrecy exponents, i.e., the exponential rates of decay of the probability of decoding error and of the information leakage. These exponents allow us to determine a set of strongly-achievable secret key rates. For degraded eavesdroppers the maximum strongly-achievable rate equals the secret key capacity; our exponents can also be specialized to previously known results. In deriving our strong achievability results we introduce a coding scheme that combines wiretap coding (to excite the channel) and key extraction (to distill keys from residual randomness). The secret key capacity is naturally seen to be a combination of both source- and channel-type randomness. Through examples we illustrate a fundamental interplay between the portion of the secret key rate due to each type of randomness. We also illustrate inherent tradeoffs between the achievable reliability and secrecy exponents. Our new scheme also naturally accommodates rate limits on the public discussion. We show that under rate constraints we are able to achieve larger rates than those that can be attained through a pure source emulation strategy.Comment: 18 pages, 8 figures; Submitted to the IEEE Transactions on Information Theory; Revised in Oct 201

Study of structure and lattice dynamics of the Sr2_2CuO2_2Cl2_2(001) surface by helium-atom scattering

Structure and lattice dynamics of the (001) surface of Sr$_2$CuO$_2$Cl$_2$ have been studied by helium atom scattering (HAS). Analysis of diffraction patterns obtained by elastic HAS revealed a surface periodicity consistent with bulk termination, and confirms that the surface is non-polar and stable which favors a SrCl surface termination. Bulk and surface lattice dynamical calculations based on the shell-model were carried out to characterize the experimental phonon dispersions obtained by inelastic HAS. No experimental surface mode was observed above 200 cm$^{-1}$. Comparison between the experimental data and theoretical results for two different slabs with SrCl and CuO$_2$ terminations showed that the experimental data conforms exclusively with the SrCl surface modes.Comment: 10 pages, 11 figure

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