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

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

Oxidative stress contributes to soluble fms-like tyrosine kinase-1 induced vascular dysfunction in pregnant rats

BACKGROUND

Recent evidence indicates that both increased oxidative stress and an altered balance between pro- and anti-angiogenic factors such as vascular-endothelial growth factor (VEGF) and the soluble VEGF receptor (sFlt-1) contribute to endothelial dysfunction in preeclampsia. We hypothesized that chronic infusion of sFlt-1 to mimic the increase observed in preeclamptic patients would reduce plasma VEGF concentrations, increase blood pressure (BP) and vascular superoxide levels, and cause endothelial dysfunction in the pregnant rat.

METHODS

Recombinant sFlt-1 was infused (500 ng/h) during days 13-18 of pregnancy. BP, fetal and placental weight, oxidative stress and vessel vasorelaxation were determined on day 18 of pregnancy.

RESULTS

Plasma sFlt-1 concentrations (299 +/- 33 vs. 100 +/- 16 pg/ml; P < 0.01) and BP (117 +/- 6 vs. 98 +/- 4 mm Hg; P < 0.01) were increased, while plasma-free VEGF concentrations (570 +/- 77 vs. 780 +/- 48 pg/ml; P < 0.01) were decreased when compared to vehicle infused dams. sFlt-1 rats had smaller fetuses (1.3 +/- 0.03 vs. 1.5 +/- 0.04 g, P < 0.01) and placentas (0.41 +/- 0.01 vs. 0.47 +/- 0.02 g; P < 0.05). Placental (180 +/- 66 vs. 24 +/- 2.3 RLU/min/mg; P < 0.05) and vascular (34 +/- 8 vs. 12 +/- 5 RLU/min/mg; P < 0.05) superoxide production was increased in the sFlt-1 compared to vehicle infused rats. Vasorelaxation to acetylecholine (ACh) and sodium nitroprusside (SNP) were both decreased (P < 0.05) in the sFlt-1 infusion group compared to the vehicle and this decrease was attenuated (P < 0.05) by the superoxide scavenger Tiron.

CONCLUSION

These data indicate elevated maternal sFlt-1 and decreased VEGF concentrations results in increased oxidative stress that contributes to vascular dysfunction during pregnancy.

Electric-field control of spin waves at room temperature in multiferroic BiFeO3

To face the challenges lying beyond present technologies based on complementary metal-oxide-semiconductors, new paradigms for information processing are required. Magnonics proposes to use spin waves to carry and process information, in analogy with photonics that relies on light waves, with several advantageous features such as potential operation in the terahertz range and excellent coupling to spintronics. Several magnonic analog and digital logic devices have been proposed, and some demonstrated. Just as for spintronics, a key issue for magnonics is the large power required to control/write information (conventionally achieved through magnetic fields applied by strip lines, or by spin transfer from large spin-polarized currents). Here we show that in BiFeO(3), a room-temperature magnetoelectric material, the spin-wave frequency (>600 GHz) can be tuned electrically by over 30%, in a non-volatile way and with virtually no power dissipation. Theoretical calculations indicate that this effect originates from a linear magnetoelectric effect related to spin-orbit coupling induced by the applied electric field. We argue that these properties make BiFeO(3) a promising medium for spin-wave generation, conversion and control in future magnonics architectures.

Light-induced size changes in BiFeO3 crystals

Multifunctional oxides are promising materials because of their fundamental physical properties as well as their potential in applications. Among these materials, multiferroics exhibiting ferroelectricity and magnetism are good candidates for spin electronic applications using the magnetoelectric effect, which couples magnetism and ferroelectricity. Furthermore, because ferroelectrics are insulators with a reasonable bandgap, photons can efficiently interact with electrons leading to photoconduction or photovoltaic effects. However, until now, coupling of light with mechanical degrees of freedom has been elusive, although ferroelasticity is a well-known property of these materials. Here, we report on the observation, for the first time, of a substantial visible-light-induced change in the dimensions of BiFeO(3) crystals at room temperature. The relative light-induced photostrictive effect is of the order of 10(-5) with response times below 0.1 s. It depends on the polarization of incident light as well as applied magnetic fields. This opens the perspective of combining mechanical, magnetic, electric and optical functionalities in future generations of remote switchable devices.

Electric field switching of the magnetic anisotropy of a ferromagnetic layer exchange coupled to the multiferroic compound BiFeO3

We report here that a Permalloy layer deposited on top of a multiferroic BiFeO3 single crystal acquires an easy magnetic direction along the propagation vector of the cycloidal arrangement of antiferromagnetic moments in BiFeO3. This anisotropy originates from a direct magnetic coupling with the canted spins forming the cycloid. Moreover, we show that an electric field-induced change of electric polarization is able to toggle the direction of anisotropy in the ferromagnet through the magnetoelectric effect, which links the antiferromagnetic spins to the local polarization in BiFeO3.

Observation of incipient charge nematicity in Ba(Fe(1-x)Co(x))2As2

Using electronic Raman spectroscopy, we report direct measurements of charge nematic fluctuations in the tetragonal phase of strain-free Ba(Fe(1-x)Co(x))2As2 single crystals. The strong enhancement of the Raman response at low temperatures unveils an underlying charge nematic state that extends to superconducting compositions and which has hitherto remained unnoticed. Comparison between the extracted charge nematic susceptibility and the elastic modulus allows us to disentangle the charge contribution to the nematic instability, and to show that charge nematic fluctuations are weakly coupled to the lattice.

Possible observation of cycloidal electromagnons in BiFeO3

We unravel the magnon spectra of BiFeO3 by means of low-energy inelastic light scattering. We show the existence of two species of magnons corresponding to spin wave excitations in and out of the cycloidal plane. These excitations might be interpreted as electromagnon modes. The present observations present an unique opportunity to study the competition between ferroelectric and magnetic orders.

Large temperature dependence of the number of carriers in co-doped BaFe(2)As(2)

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe(1-x)Co(x))(2)As(2) as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large below 100 K at low Co contents and reduces the volume of hole and electron pockets by a factor 2. This k shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations.

Optical Writing of Magnetic Properties by Remanent Photostriction

We present an optically induced remanent photostriction in BiFeO_{3}, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO_{3}/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO_{3}. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.

Nematic fluctuations in the cuprate superconductor Bi2Sr2CaCu2O8+δ

Establishing the presence and the nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, in the cuprates they remain undetected until now. Here using symmetry-resolved electronic Raman scattering in the cuprate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta}$$\end{document}Bi2Sr2CaCu2O8+δ, we report the observation of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase. While our data hint at the possible presence of an incipient nematic quantum critical point, the doping dependence of the nematic fluctuations deviates significantly from a canonical quantum critical scenario. The observed nematic instability rather appears to be tied to the presence of a van Hove singularity in the band structure.

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Collapse of the normal-state pseudogap at a Lifshitz transition in the Bi(2)Sr(2)CaCu(2)O(8+δ) cuprate superconductor

We report a fine tuned doping study of strongly overdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} single crystals using electronic Raman scattering. Combined with theoretical calculations, we show that the doping, at which the normal-state pseudogap closes, coincides with a Lifshitz quantum phase transition where the active holelike Fermi surface becomes electronlike. This conclusion suggests that the microscopic cause of the pseudogap is sensitive to the Fermi surface topology. Furthermore, we find that the superconducting transition temperature is unaffected by this transition, demonstrating that their origins are different on the overdoped side.

Cascade of bulk magnetic phase transitions in NaxCoO2 as studied by muon spin rotation

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

Hall effect and resistivity study of the magnetic transition, carrier content, and Fermi-Liquid Behavior in Ba(Fe(1-x) Co(x))(2)As(2)

The negative Hall constant R(H) measured all over the phase diagram of Ba(Fe(1-x) Co(x))(2)As(2) 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, a change with T of the electron concentration 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.

Evidence for two distinct energy scales in the Raman spectra of YBa2(Cu1-xNix)3O6.95

We report electronic Raman scattering from Ni-substituted YBa2Cu3O6.95 single crystals with T(c) ranging from 92.5 to 78 K. The fully symmetrical A(1g) channel and the B(1g) channel which is sensitive to the d(x(2)-y(2)) gap maximum have been explored. The energy of the B(1g) pair-breaking peak remains constant under Ni doping while the energy of the A(1g) peak scales with T(c) ( E(A(1g))/k(B)T(c) = 5). Our data show that the A(1g) peak tracks the magnetic resonance peak observed in inelastic neutron scattering yielding a key explanation to the long-standing problem of the origin of the A(1g) peak.

Driving Spin Excitations by Hydrostatic Pressure in BiFeO(3)

Optical spectroscopy has been combined with computational and theoretical techniques to show how the spin dynamics in the model multiferroic BiFeO(3) responds to the application of hydrostatic pressure and its corresponding series of structural phase transitions from R3c to the Pnma phases. As pressure increases, multiple spin excitations associated with noncollinear cycloidal magnetism collapse into two excitations, which show jump discontinuities at some of the ensuing crystal phase transitions. The effective Hamiltonian approach provides information on the electrical polarization and structural changes of the oxygen octahedra through the successive structural phases. The extracted parameters are then used in a Ginzburg-Landau model to reproduce the evolution with pressure of the spin wave excitations observed at low energy, and we demonstrate that the structural phases and the magnetic anisotropy drive and control the spin excitations.

Nernst effect and disorder in the normal state of high-T(c) cuprates

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-delta) single crystals. The fluctuation regime above T(c) expands significantly with disorder, indicating that the T(c) decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T(c) cuprate families. Our results show that the presence of intrinsic disorder can explain the enhanced range of the Nernst signal found in the pseudogap phase of the latter compounds.

Long-Range Order in the Dipolar XY Antiferromagnet Er_{2}Sn_{2}O_{7}

Er_{2}Sn_{2}O_{7} remains a puzzling case among the extensively studied frustrated compounds of the rare-earth pyrochlore family. Indeed, while a first-order transition towards a long-range antiferromagnetic state with the so-called Palmer-Chalker structure is theoretically predicted, it has not yet been observed, leaving the issue as to whether it is a spin-liquid candidate open. We report on neutron scattering and magnetization measurements which evidence a second-order transition towards this Palmer-Chalker ordered state around 108 mK. Extreme care was taken to ensure a proper thermalization of the sample, which has proved to be crucial to successfully observe the magnetic Bragg peaks. At the transition, a gap opens in the excitations, superimposed on a strong quasielastic signal. The exchange parameters, refined from a spin-wave analysis in applied magnetic field, confirm that Er_{2}Sn_{2}O_{7} is a realization of the dipolar XY pyrochlore antiferromagnet. The proximity of competing phases and the strong XY anisotropy of the Er^{3+} magnetic moment might be at the origin of enhanced fluctuations, leading to the unexpected nature of the transition, the low ordering temperature, and the observed multiscale dynamics.

Enhancement of the critical temperature of HgBa₂CuO(4+δ) by applying uniaxial and hydrostatic pressure: implications for a universal trend in cuprate superconductors

It is well known that the superconducting transition temperature (T(c)) of cuprate superconductors can be enhanced by varying certain structural and electronic parameters, such as the flatness of the CuO₂ planes or their doping level. We determine the uniaxial and hydrostatic pressure derivatives of T(c) in the structurally simple tetragonal compound HgBa₂CuO(4+δ) near optimal doping. Our results provide experimental evidence for two further methods to enhance T(c): (i) reducing the area of the CuO₂ planes, and (ii) increasing the separation of the CuO₂ planar groups. T(c) is found to couple much more strongly to the ratio c/a of the lattice constants than to the unit cell volume. A comparison with prior results for structurally more complicated cuprates reveals a general trend of uniaxial pressure derivatives with T(c).

Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs

The resistivity, Hall effect, and transverse magnetoresistance have been measured in low residual resistivity single crystals of LiFeAs. A comparison with angle resolved photoemission spectroscopy and quantum oscillation data implies that four carrier bands unevenly contribute to the transport. However the scattering rates of the carriers all display the T(2) behavior expected for a Fermi liquid. Near T(c) low field deviations of the magnetoresistance with respect to a H(2) variation permit us to extract the superconducting fluctuation contribution to the conductivity. Though below T(c) the anisotropy of superconductivity is rather small, the superconducting fluctuation displays a quasi-ideal two-dimensional behavior which persists up to 1.4 T(c). These results call for a refined theoretical understanding of the multiband behavior of superconductivity in this pnictide.

Total suppression of superconductivity by high magnetic fields in YBa(2)Cu(3)O(6.6)

We have studied the variation of transverse magnetoresistance of underdoped YBCO(6.6) crystals, either pure or with reduced T(c) down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H(c)'(T), which is found to vanish at T(c)' > T(c). In the pure YBCO(6.6) sample, H(c)' is already 50 T at T(c). We find that increasing disorder weakly depresses H(c)'(0), T(c)', and T(nu), the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.