2,892 research outputs found
Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B
Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2â3 x 2â3R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effectsThis work has received the financial support of the French ANR SURMOTT program (ANR-09-BLAN- 0210-01) and the Spanish MICIIN under Project No. FIS2010-1604
X-ray anomalous scattering investigations on the charge order in -NaVO
Anomalous x-ray diffraction studies show that the charge ordering in
-NaVO is of zig-zag type in all vanadium ladders. We
have found that there are two models of the stacking of layers along
\emph{c-}direction, each of them consisting of 2 degenerated patterns, and that
the experimental data is well reproduced if the 2 patterns appears
simultaneously. We believe that the low temperature structure contains stacking
faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the
tex
High field magnetic resonant properties of beta'-(ET)2SF5CF2SO3
A systematic electron spin resonance (ESR) investigation of the low
temperature regime for the (ET)2SF5CF2SO3 system was performed in the frequency
range of ~200-700 GHz, using backward wave oscillator sources, and at fields up
to 25 T. Newly acquired access to the high frequency and fields shows
experimental ESR results in agreement with the nuclear magnetic resonance (NMR)
investigation, revealing evidence that the transition seen at 20 K is not of
conventional spin-Peierls order. A significant change of the spin resonance
spectrum in beta'-(ET)2SF5CF2SO3 at low temperatures, indicates a transition
into a three-dimensional-antiferromagnetic (3D AFM) phase.Comment: 4 pages, 7 figures, minor grammatical change
Identification of Nuclear Relaxation Processes in a Gapped Quantum Magnet: Proton NMR in the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4
The proton hyperfine shift K and NMR relaxation rate have been
measured as a function of temperature in the S=1/2 Heisenberg antiferromagnetic
ladder Cu2(C5H12N2)2Cl4. The presence of a spin gap in this strongly coupled ladder ()
is supported by the K and results. By comparing at two
different proton sites, we infer the evolution of the spectral functions
and . When the gap is significantly
reduced by the magnetic field, two different channels of nuclear relaxation,
specific to gapped antiferromagnets, are identified and are in agreement with
theoretical predictions.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Letter
BaVS probed by V L edge X-ray absorption spectroscopy
Polarization dependent vanadium L edge X-ray absorption spectra of BaVS
single crystals are measured in the four phases of the compound. The difference
between signals with the polarization \textbf{E}\textbf{c} and
\textbf{E}\textbf{c} (linear dichroism) changes with temperature.
Besides increasing intensity of one of the maxima, a new structure appears in
the pre-edge region below the metal-insulator transition. More careful
examination brings to light that the changes start already with pretransitional
charge density wave fluctuations. Simple symmetry analysis suggests that the
effect is related to rearrangements in and states, and is
compatible with the formation of four inequivalent V sites along the V-S chain.Comment: 6 pages, 6 figure
NMR study of the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4 : Quantum phase transition and critical dynamics
We present an extensive NMR study of the spin-1/2 antiferromagnetic
Heisenberg ladder Cu2(C5H12N2)2Cl4 in a magnetic field range 4.5 - 16.7 T. By
measuring the proton NMR relaxation rate 1/T_1 and varying the magnetic field
around the critical field H_c1 = Delta / g\mu_B = 7.5 T, we have studied the
transition from a gapped spin liquid ground state to a gapless magnetic regime
which can be described as a Luttinger liquid. We identify an intermediate
regime T > |H-H_c1|, where the spin dynamics is (possibly) only controlled by
the T=0 critical point H_c1.Comment: 4 pages, 3 eps figures, submitted to Phys. Rev. Let
Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in -NaVO studied by far infrared spectroscopy
We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had
been los
The lady vanishes: what's missing from the stem cell debate
Most opponents of somatic cell nuclear transfer and embryonic stem cell technologies base their arguments on the twin assertions that the embryo is either a human being or a potential human being, and that it is wrong to destroy a human being or potential human being in order to produce stem cell lines. Proponentsâ justifications of stem cell research are more varied, but not enough to escape the charge of obsession with the status of the embryo. What unites the two warring sides in âthe stem cell warsâ is that women are equally invisible to both: âthe lady vanishesâ. Yet the only legitimate property in the body is that which women possess in their reproductive tissue and the products of their reproductive labour. By drawing on the accepted characterisation in law of property as a bundle of rights, and on a Hegelian model of contract as mutual recognition, we can lessen the impact of the tendency to regard women and their eggs as merely receptacles and womenâs reproductive labour as unimportant
- âŠ