70 research outputs found
Unconventional antiferromagnetic correlations of the doped Haldane gap system YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. We propose a new
mean field model relevant for this compound. The ground state of this mean
field model is unconventional because antiferromagnetism coexists with random
dimers. We present new susceptibility experiments at very low temperature. We
obtain a Curie-Weiss susceptibility as expected
for antiferromagnetic correlations but we do not obtain a direct signature of
antiferromagnetic long range order. We explain how to obtain the ``impurity''
susceptibility by subtracting the Haldane gap contribution to
the total susceptibility. In the temperature range [1 K, 300 K] the
experimental data are well fitted by . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. This fit suggests the existence of a finite N\'eel temperature which is
however too small to be probed directly in our experiments. We also obtain a
maximum in the temperature dependence of the ac-susceptibility which
suggests the existence of antiferromagnetic correlations at very low
temperature.Comment: 19 pages, 17 figures, revised version (minor modifications
Evidence of quantum criticality in the doped Haldane system Y2BaNiO5
Experimental bulk susceptibility X(T) and magnetization M(H,T) of the
S=1-Haldane chain system doped with nonmagnetic impurities, Y2BaNi1-xZnxO5
(x=0.04,0.06,0.08), are analyzed. A numerical calculation for the low-energy
spectrum of non-interacting open segments describes very well experimental data
above 4 K. Below 4 K, we observe power-law behaviors, X(T)=T^-alpha and
M(H,T)/T^(1-alpha)=f(alpha,(H/T)), with alpha (<1) depending on the doping
concentration x.This observation suggests the appearance of a gapless quantum
phase due to a broad distribution of effective couplings between the
dilution-induced moments.Comment: 4 pages, 3 figure
First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8
The nature of the Mott transition in the absence of any symmetry braking
remains a matter of debate. We study the correlation-driven insulator-to-metal
transition in the prototypical 3D Mott system GaTa4Se8, as a function of
temperature and applied pressure. We report novel experiments on single
crystals, which demonstrate that the transition is of first order and follows
from the coexistence of two states, one insulating and one metallic, that we
toggle with a small bias current. We provide support for our findings by
contrasting the experimental data with calculations that combine local density
approximation with dynamical mean-field theory, which are in very good
agreement.Comment: 5 pages and 4 figures. Supplemental material: 2 pages, 2 figure
Self-Organized Criticality Effect on Stability: Magneto-Thermal Oscillations in a Granular YBCO Superconductor
We show that the self-organized criticality of the Bean's state in each of
the grains of a granular superconductor results in magneto-thermal oscillations
preceding a series of subsequent flux jumps. We find that the frequency of
these oscillations is proportional to the external magnetic field sweep rate
and is inversely proportional to the square root of the heat capacity. We
demonstrate experimentally and theoretically the universality of this
dependence that is mainly influenced by the granularity of the superconductor.Comment: submitted to Physical Review Letters, 4 pages, RevTeX, 4 figures
available as uufile
Ultrafast filling of an electronic pseudogap in an incommensurate crystal
We investigate the quasiperiodic crystal (LaS)1.196(VS2) by angle and time
resolved photoemission spectroscopy. The dispersion of electronic states is in
qualitative agreement with band structure calculated for the VS2 slab without
the incommensurate distortion. Nonetheless, the spectra display a temperature
dependent pseudogap instead of quasiparticles crossing. The sudden
photoexcitation at 50 K induces a partial filling of the electronic pseudogap
within less than 80 fs. The electronic energy flows into the lattice modes on a
comparable timescale. We attribute this surprisingly short timescale to a very
strong electron-phonon coupling to the incommensurate distortion. This result
sheds light on the electronic localization arising in aperiodic structures and
quasicrystals
Electric Pulse Induced Resistive Switching, Electronic Phase Separation, and Possible Superconductivity in a Mott insulator
Metal-insulator transitions (MIT) belong to a class of fascinating physical
phenomena, which includes superconductivity, and colossal magnetoresistance
(CMR), that are associated with drastic modifications of electrical resistance.
In transition metal compounds, MIT are often related to the presence of strong
electronic correlations that drive the system into a Mott insulator state. In
these systems the MIT is usually tuned by electron doping or by applying an
external pressure. However, it was noted recently that a Mott insulator should
also be sensitive to other external perturbations such as an electric field. We
report here the first experimental evidence of a non-volatile
electric-pulse-induced insulator-to-metal transition and possible
superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling
Microscopy experiments show that this unconventional response of the system to
short electric pulses arises from a nanometer scale Electronic Phase Separation
(EPS) generated in the bulk material.Comment: Highlight in Advanced Functional Materials 18, 1-4 (2008) doi :
10.1002/adfm.20080055
Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales
Femtosecond time-resolved X-ray diffraction is used to study a photo-induced
phase transition between two charge density wave (CDW) states in 1T-TaS,
namely the nearly commensurate (NC) and the incommensurate (I) CDW states.
Structural modulations associated with the NC-CDW order are found to disappear
within 400 fs. The photo-induced I-CDW phase then develops through a
nucleation/growth process which ends 100 ps after laser excitation. We
demonstrate that the newly formed I-CDW phase is fragmented into several
nanometric domains that are growing through a coarsening process. The
coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth
law, which describes the ordering kinetics in systems exhibiting a
non-conservative order parameter.Comment: 6 pages, 5 figure
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