644 research outputs found
Pressure dependence of phase transitions in the quasi one-dimensional metal-insulator transition system beta-Na1/3V2O5
The pressure dependence of phase transitions in the quasi one-dimensional
vanadium oxide -NaVO has been studied by magnetic
susceptibility and electrical resistivity measurements. The pressure dependence
of the various transition temperatures is quite differently. The transition at
T=240 K, previously reported and attributed to ordering on Na sites, and a
second transition at K, reported here for the first time and
attributed to a further increase of order on Na sites, are almost independent
of pressure. On the other hand, the metal-insulator (MI) transition at
K shifts to lower temperatures, while the magnetic transition at
K shifts to higher temperatures with increasing pressure. We discuss
the different pressure dependencies of and in terms of
increasing interchain coupling and the MI transition to be of Peierls type.Comment: 5 pages, 5 figure
Structural precursor to the metal-insulator transition in V_2O_3
The temperature dependence of the local structure of V_2O_3 in the vicinity
of the metal to insulator transition (MIT) has been investigated using hard
X-ray absorption spectroscopy. It is shown that the vanadium pair distance
along the hexagonal c-axis changes abruptly at the MIT as expected. However, a
continuous increase of the tilt of these pairs sets in already at higher
temperatures and reaches its maximum value at the onset of the electronic and
magnetic transition. These findings confirm recent theoretical results which
claim that electron-lattice coupling is important for the MIT in V_2O_3. Our
results suggest that interactions in the basal plane play a decisive role for
the MIT and orbital degrees of freedom drive the MIT via changes in
hybridization.Comment: 6 pages, 5 figures, 2 table
Chain Formation by Spin Pentamers in eta-Na9V14O35
The nature of the gapped ground state in the quasi-one-dimensional compound
eta-Na9V14O35 cannot easily be understood, if one takes into account the odd
number of spins on each structural element. Combining the results of specific
heat, susceptibility and electron spin resonance measurements we show that
eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a
linear chain. These objects - pentamers - consist of five antiferromagnetically
arranged spins with effective spin 1/2. Their spatial extent results in an
exchange constant along the chain direction comparable to the one in the
high-temperature state.Comment: 6 pages, 5 figure
The NeuroDante Project: Neurometric measurements of participant’s reaction to literary auditory stimuli from dante’s “divina commedia”
Neurodante. Progetto di analisi neurometrica di alcuni brani della Commedi
Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure
We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve
structure, the resistive state of which depends on the preceding magnetic field
polarity. The effect is based on a strong exchange bias (about -2 kOe) on a
diluted ferromagnetic copper-nickel alloy and generation of a long range odd in
frequency triplet pairing component. The difference of high and low resistance
states at zero magnetic field is 90% of the normal state resistance for a
transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic
states of the structure do not require biasing fields or currents in the idle
mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter
Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 Grown on Cobalt Oxide
Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core
of a superconducting spin valve. The switching effect of the spin valve is
based on interference phenomena occurring due to the proximity effect at the
S/F interfaces. A remarkable effect is only expected if the core structure
exhibits strong critical temperature oscillations, or most favorable, reentrant
superconductivity, when the thickness of the ferromagnetic layer is increased.
The core structure has to be grown on an antiferromagnetic oxide layer (or such
layer to be placed on top) to pin by exchange bias the
magnetization-orientation of one of the ferromagnetic layers. In the present
paper we demonstrate that this is possible, keeping the superconducting
behavior of the core structure undisturbed.Comment: 22 pages, 12 figures, 1 tabl
Ferromagnetism in the large-U Hubbard model
We study the Hubbard model on a hypercubic lattice with regard to the
possibility of itinerant ferromagnetism. The Dynamical Mean Field theory is
used to map the lattice model on an effective local problem, which is treated
with help of the Non Crossing Approximation. By investigating spin dependent
one-particle Green's functions and the magnetic susceptibility, a region with
nonvanishing ferromagnetic polarization is found in the limit . The
-T-phase diagram as well as thermodynamic quantities are discussed. The
dependence of the Curie temperature on the Coulomb interaction and the
competition between ferromagnetism and antiferromagnetism are studied in the
large limit of the Hubbard model.Comment: 4 pages, 5 figures, accepted for publication in Physical Review B,
Rapid Communication
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
Re-entrant superconductivity in Nb/Cu(1-x)Ni(x) bilayers
We report on the first observation of a pronounced re-entrant
superconductivity phenomenon in superconductor/ferromagnetic layered systems.
The results were obtained using a superconductor/ferromagnetic-alloy bilayer of
Nb/Cu(1-x)Ni(x). The superconducting transition temperature T_{c} drops sharply
with increasing thickness d_{CuNi} of the ferromagnetic layer, until complete
suppression of superconductivity is observed at d_{CuNi}= 4 nm. Increasing the
Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at
d_{CuNi}=13 nm. Our experiments give evidence for the pairing function
oscillations associated with a realization of the quasi-one dimensional
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer.Comment: 3 pages, 3 figures, REVTEX4/twocolum
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