arXiv subjects

A. Forget

Publications and source records attributed to A. Forget.

At least 73 records · Page 4Linked to original sources

arXiv2010

Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.

cond-mat.supr-con↗

arXiv2009

Dynamic correlated Cu(2) magnetic moments in superconducting YBa_2(Cu_0.96Ni_0.04)_3O_y (y ~ 7)

We have examined the magnetic properties of polycrystalline, superconducting YBa_2(Cu_0.96Ni_0.04)_3O_y (y ~ 7, T_sc ~ 75 K) using two local probe techniques: 170Yb Moessbauer down to 0.1 K and muon spin relaxation (muSR) down to 1.5 K. At 0.1 K, the 170Yb measurements show the Cu(2) over essentially all the sample volume carry magnetically correlated moments which are static on the time-scale of 10^{-9} s. The moments show a distribution in size. The correlations are probably short range. As the temperature increases, the correlated moments are observed to fluctuate with measurable rates (in the GHz range) which increase as the temperature increases and which show a wide distribution. The muSR measurements also evidence that the fluctuation rates increase with increasing temperature and there is a distribution. The evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.

cond-mat.str-el↗

arXiv2009

Atomic coexistence of superconductivity and incommensurate magnetic order in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below T=31K. Comparison with undoped compound allows to estimate a typical moment of 0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of 0.04. Below TC=21.8K, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remains unaffected, demonstrating coexistence of both states on each Fe site.

cond-mat.supr-con↗

arXiv2009

Magnetization and magneto-electric effect in La-doped BiFeO3

We report on magnetisation and magneto-capacitance measurements in the Bi1-xLaxFeO3 series for 0 < x < 0.15. We confirm that doping with La reduces the threshold magnetic field Hc for cancelling the magnetic spiral phase, and we show that Hc decreases as the La content increases up to x=0.15, which is the highest concentration for maintaining the non-centrosymmetric rhombohedral structure of BiFeO3. Measurements of the dielectric constant as a function of magnetic field in the series also show a maximum magneto-capacitance for x=0.15.

cond-mat.str-el↗

arXiv2009

Hall effect and resistivity study of the magnetic transition, carrier content and Fermi liquid behavior in Ba(Fe(1-x) Cox)2As2

The negative Hall constant R_H measured all over the phase diagram of Ba(Fe(1-x)Cox)2As2 allows us to show that electron carriers always dominate the transport properties. The evolution of R_H with x at low doping (x<2%) indicates that important band structure changes happen for x<2% prior to the emergence of superconductivity. For higher x, an change of the electron concentration with T is required to explain the low T variations of R_H, while the electron scattering rate displays the T^2 law expected for a Fermi liquid. The T=0 residual scattering is affected by Co disorder in the magnetic phase, but is rather dominated by incipient disorder in the paramagnetic state.

cond-mat.supr-con↗

arXiv2009

Nesting between hole and electron pockets in Ba(Fe1-xCox)2As2 (x=0-0.3) observed with angle-resolved photoemission

We present a comprehensive angle-resolved photoemission study of the three-dimensional electronic structure of Ba(Fe1-xCox)2As2. The wide range of dopings covered by this study, x=0 to x=0.3, allows to extract systematic features of the electronic structure. We show that there are three different hole pockets around the G point, the two inner ones being nearly degenerate and rather two dimensional, the outer one presenting a strong three dimensional character. The structure of the electron pockets is clarified by studying high doping contents, where they are enlarged. They are found to be essentially circular and two dimensional. From the size of the pockets, we deduce the number of holes and electrons present at the various dopings. We find that the net number of carriers is in good agreement with the bulk stoichiometry, but that the number of each species (holes and electrons) is smaller than predicted by theory. Finally, we discuss the quality of nesting in the different regions of the phase diagram. The presence of the third hole pocket significantly weakens the nesting at x=0, so that it may not be a crucial ingredient in the formation of the Spin Density Wave. On the other hand, superconductivity seems to be favored by the coexistence of two-dimensional hole and electron pockets of similar sizes.

cond-mat.str-el↗

arXiv2009

Magnetic order in Tb$_2$Sn$_2$O$_7$ under high pressure: from ordered spin ice to spin liquid and antiferromagnetic order

We have studied the Tb$_2$Sn$_2$O$_7$ frustrated magnet by neutron diffraction under isotropic pressure of 4.6 GPa, combined with uniaxial pressure of 0.3 GPa, in the temperature range 0.06 K$<$T$<$100 K. Magnetic order persists under pressure but the ordered spin ice structure stabilized at ambient pressure below 1.3 K partly transforms into an antiferromagnetic one. The long range ordered moment at 0.06 K is reduced under pressure, which is interpreted by a pressure induced enhancement of the spin liquid fluctuations. Above the ordering transition, short range spin correlations are affected by pressure, and ferromagnetic correlations are suppressed. The influence of pressure on the ground state is discussed considering both isotropic and stress effects.

cond-mat.str-el↗

arXiv2009

Doping dependence of the lattice dynamics in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ studied by Raman spectroscopy

We report Raman scattering spectra of iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with varying cobalt $x$ content. Upon cooling through the tetragonal-to-orthorhombic transition, we observe a large splitting of the E$_g$ in-plane phonon modes involving Fe and As displacements. The splitting of the in-plane phonons at the transition is strongly reduced upon doping and disappears for $x=0.06$ qualitatively following the trend displayed by the Fe magnetic moment. The origin of the splitting is discussed in terms of magnetic frustration inherent to iron-pnictide systems and we argue that such enhanced splitting may be linked to strong spin-phonon coupling.

cond-mat.str-el↗

arXiv2009

Spin-lattice coupling, frustration and magnetic order in multiferroic RMnO3

We have performed high resolution neutron diffraction and inelastic neutron scattering experiments in the frustrated multiferroic hexagonal compounds RMnO3 (R=Ho, Yb, Sc, Y), which provide evidence of a strong magneto-elastic coupling in the the whole family. We can correlate the atomic positions, the type of magnetic structure and the nature of the spin waves whatever the R ion and temperature. The key parameter is the position of the Mn ions in the unit cell with respect to a critical threshold of 1/3, which determines the sign of the coupling between Mn triangular planes.

cond-mat.str-el↗

arXiv2009

Spin Dynamics in Cuprates: Optical Conductivity of HgBa2CuO4

The electron-boson spectral density function I^2ChiOmega responsible for carrier scattering of the high temperature superconductor HgBa2CuO4 (Tc = 90 K) is calculated from new data on the optical scattering rate. A maximum entropy technique is used. Published data on HgBa2Ca2Cu3O8 (Tc = 130 K) are also inverted and these new results are put in the context of other known cases. All spectra (with two notable exceptions) show a peak at an energy (Omega_r) proportional to the superconducting transition temperature Omega_r ~= 6.3 kB.Tc. This charge channel relationship follows closely the magnetic resonance seen by polarized neutron scattering, Omega_r^{neutron} ~= 5.4 kB.Tc. The amplitudes of both peaks decrease strongly with increasing temperature. In some cases, the peak at Omega_r is weak and the spectrum can have additional maxima and a background extending up to several hundred meV.

cond-mat.supr-con↗

arXiv2008

Investigation of the magnetic fluctuations in Tb$_2$Sn$_2$O$_7$ ordered spin ice by high resolution energy-resolved neutron scattering

We have studied magnetically frustrated Tb$_2$Sn$_2$O$_7$ by neutron diffraction and high resolution energy-resolved neutron scattering. At 0.1 K, we observe short range magnetic correlations with a typical scale of 4 Å, close to the near neighbor distance between Tb$^{3+}$ ions. %(3.686 Å), This short range order coexists with ferromagnetic correlations and long range spin ice order at the scales of 18 and 190 Å, respectively. Spin dynamics was investigated at a time scale down to 10$^{-9}$s, by energy-resolved experiments on a backscattering spectrometer. We observe a freezing of the spin dynamics for all length-scales, with a strong slowing down of the spin fluctuations when long range order settles in. We discuss the spin fluctuations remaining in the ground state in comparison with previous data obtained by muon spectroscopy.

cond-mat.str-el↗

arXiv2008

Magnetic order in YbMnO$_3$ studied by Neutron Diffraction and Mössbauer Spectroscopy

The magnetic ordering of the hexagonal multiferroic compound YbMnO$_3$ has been studied between 100 K and 1.5 K by combining neutron powder diffraction, $^{170}$Yb Mössbauer spectroscopy and magnetization measurements. The Yb moments of the two crystallographic sites order at two different temperatures, the $4b$ site together with the Mn moments (at $T_N \simeq$85 K) and the $2a$ site well below (at 3.5 K). The temperature dependences of the Mn and Yb moments are explained within a molecular field model, showing that the $4b$ and $2a$ sites order via Yb-Mn and Yb-Yb interactions respectively. A simple picture taking into account the local Mn environment of the Rare earth R ($4b$) ion is proposed to couple R and Mn orders in hexagonal RMnO$_3$ manganites. The nature and symmetry of the R-Mn interactions yielding the R order are discussed.

cond-mat.str-el↗

arXiv2008

The nodal gap component as a good candidate for the superconducting order parameter in cuprates

Although more than twenty years have passed since the discovery of high temperature cuprate superconductivity, the identification of the superconducting order parameter is still under debate. Here, we show that the nodal gap component is the best candidate for the superconducting order parameter. It scales with the critical temperature $T_c$ over a wide doping range and displays a significant temperature dependence below $T_c$ in both the underdoped and the overdoped regimes of the phase diagram. In contrast, the antinodal gap component does not scale with $T_c$ in the underdoped side and appears to be controlled by the pseudogap amplitude. Our experiments establish the existence of two distinct gaps in the underdoped cuprates.

cond-mat.supr-con↗

arXiv2008

Electric-field-induced spin-flop in BiFeO3 single crystals at room-temperature

Bismuth ferrite, BiFeO3, is the only known room-temperature 'multiferroic' material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric orders are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarisation re-orientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than that observed in thin films where the cycloid is absent.

cond-mat.mtrl-sci↗

arXiv2007

Investigations on the relationship between $T_c$ and the superconducting gap under magnetic and non-magnetic impurity substitutions in YBa$_2$Cu$_3$O$_{7-δ}$

We report electronic Raman scattering measurements on optimally doped YBa2Cu3O7 where Zn or Ni impurities have been substituted by Cu. Using Raman selection rules, we have probed the superconducting gap in the nodal and antinodal regions. We show that under impurity substitutions, the energy of the antinodal peak detected in the superconducting state is not related to the critical temperature Tc and that signatures of superconductivity disappear in the nodal regions. Our experimental findings advocate in favor of gapless arcs around the nodes. The breakdown of the relationship between the antinodal gap amplitude and Tc is discussed in terms of local superconducting gap and pseudogap.

cond-mat.supr-con↗

arXiv2007

Evolution of the gaps through the cuprate phase-diagram

The actual physical origin of the gap at the antinodes, and a clear identification of the superconducting gap are fundamental open issues in the physics of high-$T_c$ superconductors. Here, we present a systematic electronic Raman scattering study of a mercury-based single layer cuprate, as a function of both doping level and temperature. On the deeply overdoped side, we show that the antinodal gap is a true superconducting gap. In contrast, on the underdoped side, our results reveal the existence of a break point close to optimal doping below which the antinodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal state is distinctly different on each side of this breakpoint.

cond-mat.supr-con↗

arXiv2007

Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields

Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow us to measure a large spontaneous polarization reaching 100 microC.cm^{-2}, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.

cond-mat.mtrl-sci↗

arXiv2006

Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6

We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc reduced down to 3.5K by electron irradiation. We evidence that the normal state MR is restored above a threshold field H'c(T), which is found to vanish at T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to completely suppress the superconducting fluctuations at Tc. While disorder does not depress the pseudogap temperature, it reduces drastically the phase coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the Nernst signal which are more characteristic of the 2D local pairing.

cond-mat.supr-con↗