794 research outputs found
Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba_0.7K_0.3Fe_2As_2: NMR in a single crystal
We report the first ^{75}As-NMR study on a single crystal of the hole-doped
iron-pnictide superconductor Ba_{0.7}K_{0.3}Fe_2As_{2} (T_c = 31.5 K). We find
that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker
compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin
lattice relaxation rate 1/T_1 decreases below T_c with no coherence peak and
shows a step-wise variation at low temperatures, which is indicative of
multiple superconducting gaps, as in the electron-doped
Pr(La)FeAsOF. Furthermore, no evidence was obtained for a
microscopic coexistence of a long-range magnetic and superconductivity
Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2
75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole
resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P =
4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice
relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak
just below Tc(P) and decrease with decreasing temperature. The
superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple
gaps at P = 10.8 kbar. We find that the superconductivity appears near a
quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar.
Both electron correlation and superconductivity disappear in the collapsed
tetragonal phase. A systematic study under pressure indicates that electron
correlations play a vital role in forming Cooper pairs in this compound.Comment: 5pages, 5figure
Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O
We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on
the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under
hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure
at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the
decrease of T_c is accompanied by a weakening of the spin correlations at a
finite wave vector and a reduction of the density of states (DOS) at the Fermi
level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly
due to a reduction of the DOS. These results indicate that the
electronic/magnetic state of Co is primarily responsible for the
superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a
T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling
evidence for the presence of line nodes in the superconducting gap function.Comment: published on 19, Sept. 2007 on Phys. Rev.
Hydration-induced anisotropic spin fluctuations in Na_{x}CoO_{2}\cdot1.3H_{2}O superconductor
We report ^{59}Co NMR studies in single crystals of cobalt oxide
superconductor Na_{0.42}CoO_{2}\cdot1.3H_{2}O (T_c=4.25K) and its parent
compound Na_{0.42}CoO_{2}. We find that both the magnitude and the temperature
(T) dependence of the Knight shifts are identical in the two compounds above
T_c. The spin-lattice relaxation rate (1/T_1) is also identical above T_0
\sim60 K for both compounds. Below T_0, the unhydrated sample is found to be a
non-correlated metal that well conforms to Fermi liquid theory, while spin
fluctuations develop in the superconductor. These results indicate that water
intercalation does not change the density of states but its primary role is to
bring about spin fluctuations. Our result shows that, in the hydrated
superconducting compound, the in-plane spin fluctuation around finite wave
vector is much stronger than that along the c-axis, which indicates that the
spin correlation is quasi-two-dimensional.Comment: 4 pages, 5 figure
Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog
We present extensive 75As NMR and NQR data on the superconducting arsenides
PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and
Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel
analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the
superconducting gap is shown to be isotropic, the spin lattice relaxation rate
1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a
step-wise variation at low temperatures. The Knight shift decreases below Tc
and shows a step-wise T variation as well. These results indicate spinsinglet
superconductivity with multiple gaps in the Fe-arsenides. The Fe
antiferromagnetic spin fluctuations are anisotropic and weaker compared to
underdoped copper-oxides or cobalt-oxide superconductors, while there is no
significant electron correlations in LaNiAsO0.9F0.1. We will discuss the
implications of these results and highlight the importance of the Fermi surface
topology.Comment: 6 pages, 11 figure
Manipulating the nematic director by magnetic fields in the spin-triplet superconducting state of CuxBi2Se3
Electronic nematicity, a consequence of rotational symmetry breaking, is an
emergent phenomenon in various new materials. In order to fully utilize the
functions of these materials, ability of tuning them through a knob, the
nematic director, is desired. Here we report a successful manipulation of the
nematic director, the vector order-parameter (d-vector), in the spin-triplet
superconducting state of CuxBi2Se3 by magnetic fields. At H = 0.5 T, the ac
susceptibility related to the upper critical field shows a two-fold symmetry in
the basal plane. At H = 1.5 T, however, the susceptibility shows a six-fold
symmetry, which has never been reported before in any superconductor. These
results indicate that the d-vector initially pinned to a certain direction is
unlocked by a threshold field to respect the trigonal crystal symmetry. We
further reveal that the superconducting gap in different crystals converges to
p_x symmetry at high fields, although it differs at low fields.Comment: Errors in figures corrected. Published versio
Analisi del campo di deformazione superficiale della Regione Campania da dati SAR (PS-InSAR)
Il lavoro presenta un’applicazione della tecnica dei diffusori permanenti (Permanent Scatterers) finalizzata alla caratterizzazione del campo di deformazione superficiale della Regione Campania. Per effettuare l’analisi sono state processate, tramite la tecnica PS-InSARTM, 1078 scene SAR ERS1 ed ERS2 relative al periodo giugno 1992 - gennaio 2001. Sulla base della velocità media annua di spostamento di 1.691.740 PS con coerenza maggiore di 0.65 sono state prodotte mappe di velocità media di deformazione mediante l’utilizzo di una procedura articolata nei seguenti punti: a) filtraggio dei dati effettuato sulla base dei valori di coerenza; b) produzione di mappe di velocità media di deformazione in coordinate SAR; c) generazione di mappe di velocità media di deformazione secondo le componenti verticale ed Est-Ovest; d) validazione di risultati mediante confronto con dati prodotti con differenti metodiche d’indagine. L’analisi dei risultati ottenuti ha consentito di identificare aree caratterizzate da distinti stili deformativi. Aree in subsidenza sono risultate essere, per il periodo di interesse, oltre all’area vulcanica dei Campi Flegrei, le aree costiere dei bacini dei fiumi Volturno e Sele ed il settore centrale dell’isola d’Ischia. Aree con significativi valori di sollevamento sono localizzate nel settore settentrionale del Monte Somma, nel settore centrale della Piana Campana, nel Sannio e nel Cilento. Significativi valori di velocità di spostamento in direzione EW si riscontrano, oltre che nell’area dei Campi Flegrei, nel settore appenninico del bacino del Sele e nell’area interposta tra il Somma-Vesuvio e la catena appenninica. Il quadro deformativo globale risulta essere caratterizzato da un andamento complesso, governato da una molteplicità di processi di differente natura (tettonici, vulcano-tettonici, esogeni ed antropici)
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