24 research outputs found
Magnetic states of lightly hole-doped cuprates in the clean limit as seen via zero-field muon spin spectroscopy
No full textWe have performed extensive zero-field muSR experiments on pure YBa2Cu3O6+y and diluted Y-rare-earth
substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent
disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the
concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant
antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment
m(h,T) fit a model with a spin activation energy for the crossover between the two regimes throughout the
entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the
re-entrant regime (0<h<0.056) and in the so-called cluster spin glass state coexisting with superconductivity
(0.056<h<0.08). We argue a common magnetic ground state for these two doping regions and dub it frozen
antiferromagnet. Conversely either frustration or finite-size effects prevail in the thermally activated antiferromagnetic
state, that vanishes at the same concentration where superconductivity emerges, suggesting the
presence of a quantum critical point at hc=0.056(2
Experimental evidence of two distinct charge carriers in underdoped cuprate superconductors
No full textWe present the results on heavily underdoped Y(1-x)Ca(x)Ba(2)Cu(3)O(6+y), which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to high-temperature transport. We argue that this hole tagging is a signature of the complexities of single-hole doping in Mott insulators, and it calls for a subtler description of the correlated bands than the usual one. We also map in great detail the underdoped superconducting phase diagram T(c) vs hole doping, which shows that the total number of mobile holes is not the driving parameter for superconductivity
Competing orders suppressed by disorder around a hidden quantum critical point in high-T-c cuprate superconductors
No full textWe report extensive muon-spin-rotation measurements on the lightly doped Y1-xCaxBa2Cu3O6+y compound, which allows us to disentangle the effect of disorder, controlled by random Ca2+ substitution, from that of mere doping. A three-dimensional phase diagram of lightly doped cuprates is accurately drawn. It shows a quantum critical point around which a thermally activated antiferromagnetic phase competes with superconductivity. Disorder suppresses both the competing order parameters and the quantum critical point, unveiling an underlying frozen state
Effect of the double doping mechanism on the phase diagram of Y1-xCaxBa2Cu3O6+y.
No full textWe investigated the effect of hole doping and quenched disorder on the phase diagram of Y1-xCaxBa2Cu3O6+y, by exploiting the double doping mechanism of Y–Ca substitution and O intercalation in the undoped-to-heavily underdoped regime. We show that the insulator to metal transition, governed by the mobile hole concentration, reflects only the charges transferred by chain oxygen (the y fraction in the chemical formula). The transition is preceded by the suppression of antiferromagnetic order, which is replaced by a cluster spin glass ground state. We discuss the effect of doping and disorder on both magnetic states and on the appearance of superconductivit
Effect of the double doping mechanism on the phase diagram of Y1-xCaxBa2Cu3O6+y
No full textWe investigated the effect of hole doping and quenched disorder on the phase diagram of Y1-xCaxBa2Cu3O6+y, by exploiting the double doping mechanism of Y–Ca substitution and O intercalation in the undoped-to-heavily underdoped regime. We show that the insulator to metal transition, governed by the mobile hole concentration, reflects only the charges transferred by chain oxygen (the y fraction in the chemical formula). The transition is preceded by the suppression of antiferromagnetic order, which is replaced by a cluster spin glass ground state. We discuss the effect of doping and disorder on both magnetic states and on the appearance of superconductivit
Spin and charge dynamics in [TbPc2]0 and [DyPc2]0 single-molecule magnets
No full textMagnetization, ac-susceptibility, and muon spin-relaxation mSR measurements have been performed in
neutral phthalocyaninato lanthanide [LnPc2]0 single-molecule magnets in order to determine the low-energy
levels structure and to compare the low-frequency spin excitations probed by means of macroscopic techniques,
such as ac susceptibility, with the ones explored by means of techniques of microscopic character, such
as mSR. Both techniques show a high temperature thermally activated regime for the spin dynamics and a
low-temperature tunneling one. While in the activated regime the correlation times for the spin fluctuations
estimated by ac susceptibility and mSR basically agree, clear discrepancies are found in the tunneling regime.
In particular, mSR probes a faster dynamics with respect to ac susceptibility. It is argued that the tunneling
dynamics probed by mSR involves fluctuations which do not yield a net change in the macroscopic magnetization
probed by ac susceptibiliy. Finally resistivity measurements in [TbPc2]0 crystals show a high temperature
nearly metallic behavior and a low-temperature-activated behavior
Singling out the effect of quenched disorder in the phase diagram of cuprates
No full textWe investigate the specifc influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of Y z Eu 1-z Ba 2 Cu 3 O 6+y samples at fxed oxygen content y = 0.35, in the range 0 < z <1. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with Néel transition TN = 320 K at z = 0 to a bulk superconductor with superconducting critical temperature Tc = 18 K at z = 1, YBa 2 Cu 3 O 6.35 . The electronic properties are fnely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O 2 bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we determine the influence of pure structural disorder connected with the Y/Eu alloy
Strain accommodation through facet matching in La1.85Sr0.15CuO4/Nd1.85Ce0.15CuO4 ramp-edge junctions
No full textScanning nano-focused X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy are used to investigate the crystal structure of ramp-edge junctions between superconducting electron-doped Nd1.85Ce0.15CuO4 and superconducting hole-doped La1.85Sr0.15CuO4 thin films, the latter being the top layer. On the ramp, a new growth mode of La1.85Sr0.15CuO4 with a 3.3° tilt of the c-axis is found. We explain the tilt by developing a strain accommodation model that relies on facet matching, dictated by the ramp angle, indicating that a coherent domain boundary is formed at the interface. The possible implications of this growth mode for the creation of artificial domains in morphotropic materials are discussed
Singling out the effect of quenched disorder in the phase diagram of cuprates
No full textWe investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of [Formula: see text] samples at fixed oxygen content y = 0.35, in the range [Formula: see text]. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with Néel transition [Formula: see text] K at z = 0 to a bulk superconductor with superconducting critical temperature [Formula: see text] K at z = 1, [Formula: see text]. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O2 bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we determine the influence of pure structural disorder connected with the Y/Eu alloy
Direct observation of nanoscale interface phase in the superconducting chalcogenide KxFe2−ySe2 with intrinsic phase separation
No full textWe have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2-ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T-c similar to 32 K), phase separation temperature (T-ps similar to 520 K), and iron-vacancy order temperature (T-vo similar to 580 K). In addition to the iron-vacancy ordered tetragonalmagnetic phase and orthorhombicmetallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below similar to 300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in themajority texturewith largemagnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T-ps and T-c is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KxFe2-ySe2
