47,720 research outputs found
IPA-CuCl: a S=1/2 Ladder with Ferromagnetic Rungs
The spin gap material IPA-CuCl3 has been extensively studied as a
ferromagnetic-antiferromagnetic bondalternating S = 1/2 chain. This description
of the system was derived from structural considerations and bulk measurements.
New inelastic neutron scattering experiments reveal a totally different
picture: IPA-CuCl3 consists of weakly coupled spin ladders with
antiferromagnetic legs and ferromagnetic rungs. The ladders run perpendicular
to the originally supposed bondalternating chain direction. The ferromagnetic
rungs make this system equivalent to a Haldane S = 1 antiferromagnet. With a
gap energy of 1.17(1) meV, a zone-boundary energy of 4.1(1) meV, and almost no
magnetic anisotropy, IPA-CuCl3 may the best Haldane-gap material yet, in terms
of suitability for neutron scattering studies in high magnetic fields.Comment: 2 pages, 2 figures, submitted to proceedings of LT24, Orlando, FL,
August 200
Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition
Temperature-dependent resistivity of graphene grown by chemical vapor
deposition (CVD) is investigated. We observe in low mobility CVD graphene
device a strong insulating behavior at low temperatures and a metallic behavior
at high temperatures manifesting a non-monotonic in the temperature dependent
resistivity.This feature is strongly affected by carrier density modulation. To
understand this anomalous temperature dependence, we introduce thermal
activation of charge carriers in electron-hole puddles induced by randomly
distributed charged impurities. Observed temperature evolution of resistivity
is then understood from the competition among thermal activation of charge
carriers, temperature-dependent screening and phonon scattering effects. Our
results imply that the transport property of transferred CVD-grown graphene is
strongly influenced by the details of the environmentComment: 7 pages, 3 figure
Minimal Theoretical Uncertainties in Inflationary Predictions
During inflation, primordial energy density fluctuations are created from
approximate de Sitter vacuum quantum fluctuations redshifted out of the horizon
after which they are frozen as perturbations in the background curvature. In
this paper we demonstrate that there exists an intrinsic theoretical
uncertainty in the inflationary predictions for the curvature perturbations due
to the failure of the well known prescriptions to specify the vacuum uniquely.
Specifically, we show that the two often used prescriptions for defining the
initial vacuum state -- the Bunch-Davies prescription and the adiabatic vacuum
prescription (even if the adiabaticity order to which the vacuum is specified
is infinity) -- fail to specify the vacuum uniquely in generic inflationary
spacetimes in which the total duration of inflation is finite. This conclusion
holds despite the absence of any trans-Planckian effects or effective field
theory cutoff related effects. We quantify the uncertainty which is applicable
to slow roll inflationary scenarios as well as for general FRW spacetimes and
find that the uncertainty is generically small. This uncertainty should be
treated as a minimal uncertainty that underlies all curvature perturbation
calculations.Comment: LaTeX file, 35 pages; some typos correcte
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