Tuning the Multiferroic Properties of BiFeO_{3} under Uniaxial Strain.

More than twenty years ago, multiferroic compounds combining in particular magnetism and ferroelectricity were rediscovered. Since then, BiFeO_{3} has emerged as the most outstanding multiferroic by combining at room temperature almost all the fundamental or applicative properties that may be desired: electroactive spin wave excitations called electromagnons, conductive domain walls, or a low band gap of interest for magnonic devices. All these properties have so far only been discontinuously strain engineered in thin films according to the lattice parameter imposed by the substrate. Here we explore the ferroelectricity and the dynamic magnetic response of BiFeO_{3} bulk under continuously tunable uniaxial strain. Using elasto-Raman spectroscopy, we show that the ferroelectric soft mode is strongly enhanced under tensile strain and driven by the volume preserving deformation at low strain. The magnonic response is entirely modified with low energy magnon modes being suppressed for tensile strain above pointing out a transition from a cycloid to an homogeneous magnetic state. Effective Hamiltonian calculations show that the ferroelectric and the antiferrodistortive modes compete in the tensile regime. In addition, the homogeneous antiferromagnetic state becomes more stable compared to the cycloidal state above a +2% tensile strain close to the experimental value. Finally, we reveal the ferroelectric and magnetic orders of BiFeO_{3} under uniaxial strain and how the tensile strain allows us to unlock and to modify in a differentiated way the polarization and the magnetic structure.

Gate-Dependent Electronic Raman Scattering in Graphene.

We report the direct observation of polarization resolved electronic Raman scattering in a gated monolayer graphene device. The evolution of the electronic Raman scattering spectra with gate voltage and its polarization dependence are in full agreement with theoretical expectations for nonresonant Raman processes involving interband electron-hole excitations across the Dirac cone. We further show that the spectral dependence of the electronic Raman scattering signal can be simply described by the dynamical polarizability of graphene in the long wavelength limit. The possibility to directly observe Dirac fermion excitations in graphene opens the way to promising Raman investigations of electronic properties of graphene and other 2D crystals.

Unconventional High-Energy-State Contribution to the Cooper Pairing in the Underdoped Copper-Oxide Superconductor HgBa_{2}Ca_{2}Cu_{3}O_{8+δ}.

We study the temperature-dependent electronic B_{1g} Raman response of a slightly underdoped single crystal HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} with a superconducting critical temperature T_{c}=122 K. Our main finding is that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at T_{c}. This result reveals a key aspect of the unconventional pairing mechanism: spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the B_{1g} Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting gap and the pseudogap.

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.

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.

Symmetry of the excitations in the hidden order state of URu2Si2.

We perform polarized electronic Raman scattering on URu2Si2 single crystals at low temperature down to 8 K in the hidden-order state and under a magnetic field up to 10 T. The hidden-order state is characterized by a sharp excitation at 1.7 meV and a gap in the electronic continuum below 6.8 meV. Both Raman signatures are of pure A2g symmetry. By comparing the behavior of the Raman sharp excitation and the neutron resonance at Q0=(0,0,1), we provide new evidence, constrained by selection rules of the two probes, that the hidden-order state breaks the translational symmetry along the c axis such that Γ and Z points fold on top of each other. The observation of these distinct Raman features with a peculiar A2g symmetry as a signature of the hidden-order phase places strong constraints on current theories of the hidden-order in URu2Si2.

New insights into the phase diagram of the copper oxide superconductors from electronic Raman scattering.

The mechanism of unconventional superconductivity is still unknown despite over 25 years passing since the discovery of high-T(c) cuprate superconductors by Bednorz and Muller (1986 Z. Phys. B 64 189). Here, we explore the cuprate phase diagram by electronic Raman spectroscopy and shed light on the superconducting state in hole-doped curates, namely, how superconductivity and the critical temperature T(c) are affected by the pseudogap.

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.

Raman-scattering measurements and theory of the energy-momentum spectrum for underdoped Bi2Sr2CaCuO(8+δ) superconductors: evidence of an s-wave structure for the pseudogap.

We reveal the full energy-momentum structure of the pseudogap of underdoped high-Tc cuprate superconductors. Our combined theoretical and experimental analysis explains the spectral-weight suppression observed in the B2g Raman response at finite energies in terms of a pseudogap appearing in the single-electron excitation spectra above the Fermi level in the nodal direction of momentum space. This result suggests an s-wave pseudogap (which never closes in the energy-momentum space), distinct from the d-wave superconducting gap. Recent tunneling and photoemission experiments on underdoped cuprates also find a natural explanation within the s-wave pseudogap scenario.

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.

Magnetic field induced dehybridization of the electromagnons in multiferroic TbMnO3.

We have studied the impact of the magnetic field on the electromagnon excitations in TbMnO3 crystal. Applying a magnetic field along the c axis, we show that the electromagnons transform into pure antiferromagnetic modes, losing their polar character. Entering in the paraelectric phase, we are able to track the spectral weight transfer from the electromagnons to the magnon excitations and we discuss the magnetic excitations underlying the electromagnons. We also point out the phonons involved in the phase transition process. This reveals that the Mn-O distance plays a key role in understanding the ferroelectricity and the polar character of the electromagnons. Magnetic field measurements along the b axis allow us to detect a new electromagnon resonance in agreement with a Heisenberg model.

Possible observation of the signature of the bad metal phase and its crossover to a Fermi liquid in κ-(BEDT-TTF)2Cu(NCS)2 bulk and nanoparticles by Raman scattering.

κ-(BEDT-TTF)2Cu(NCS)2 has been investigated by Raman scattering in both bulk and nanoparticle compounds. Phonon modes from 20 to 1600 cm-1 have been assigned. Focusing on the unexplored low frequency phonons, a plateau in frequencies is observed in the bulk phonons between 50 and 100 K and assigned to the signature of the bad metal phase. Nanoparticles of κ-(BEDT-TTF)2Cu(NCS)2 exhibit anomalies at 50 K associated to the crossover from a bad metal to a Fermi liquid whose origins are discussed.

Elastic and magnetoelastic properties of TbMnO3single crystal by nanosecond time resolved acoustics and first-principles calculations.

Time resolved pump and probe acoustics and first-principles calculations were employed to assess elastic properties of the TbMnO3perovskite manganite having orthorhombic symmetry. Measuring sound velocities of bulk longitudinal and shear acoustic waves propagating along at least two different directions in the high symmetry planes (100), (010) and (001), provided a powerful mean to selectively determine the six diagonal elastic constantsC11= 227 GPa,C22= 349 GPa,C33= 274 GPa,C44= 71 GPa,C55= 57 GPa,C66= 62 GPa. Among the three remaining off-diagonal ones,C23= 103 GPa was determined with a bissectrice direction. Density functional theory calculations with colinear spin-polarized provided complementary insights on their optical, elastic and magnetoelastic properties.

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 [Formula: see text], 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.

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3.

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

Delivery management in a woman with thrombocytopenia of the May-Hegglin anomaly type.

Thrombocytopenia of the May-Hegglin anomaly type was diagnosed in a woman with no past history of bleeding diathesis, who had been followed during her three pregnancies. No abnormal bleeding occurred although no platelet transfusion was administered during the second and third cesarean sections. Routine platelets transfusion is unnecessary but platelets should be available for use if abnormal bleeding occurs.

Treatment of resistant ventricular tachycardia with endocavitary fulguration and antiarrhythmic therapy, compared to antiarrhythmic therapy alone: experience in 111 consecutive cases with a mean follow-up of 18 months.

Endocavitary fulguration was performed in 31 cases of high-risk ventricular tachycardia (VT) for which antiarrhythmics, including Amiodarone and Class-I antiarrhythmic agents, given alone or in combination, proved ineffective. Permanent VT was incessant in nine patients at the time of fulguration: five of these were moribund, and two were unconscious. Included in the series were ten cases of arrhythmogenic right ventricular dysplasia, 13 cases of chronic VT after myocardial infarction, five cases of idiopathic dilated cardiomyopathy, two cases of idiopathic VT, and one case that involved a congenital anomaly. Combined fulguration and antiarrhythmic therapy succeeded in preventing VT in the 27 patients who survived the initial period of treatment. Thirteen patients needed two or more sessions, however, before their VT was brought under control. Three of the four early deaths were probably related to an imperfect technique. Four late deaths were due to spontaneous evolution of the disease. One patient without cardiac failure died suddenly. The follow-up period ranged from a minimum of 13 to a maximum of 34 months, with an average follow-up of nearly 2 years. The success rate for fulguration alone or for antiarrhythmic drugs is about 90%. This group of patients is compared with a group that involved 73 less severe cases treated with drugs. Fulguration appears to be the better form of treatment.

[Treatment of ventricular tachycardia by endocardial fulguration. Apropos of 86 cases]. / Le traitement des tachycardies ventriculaires par la fulguration endocavitaire. A propos de 86 cas.

Between 1983 and 1991, 86 cases of ventricular tachycardia (VT) resistant to antiarrhythmic therapy were treated by endocardious catheter fulguration: there were 21 cases of right ventricular dysplasia (RVD), 35 chronic myocardial infarctions (MI), 11 dilated cardiomyopathies (DCM), 10 bundle branch VT, 5 idiopathic septal VT, 3 operated Fallot procedures, 1 Ebstein's anomaly. There were 69 men and 17 women aged 14 to 76 years (average 45 +/- 18 years). The ejection fraction was under 30% in 37 cases. Forty-five VTs were permanent or recurred several times daily; 6 were inducible despite drug therapy and 35 patients had monthly recurrences. A total of 141 different forms of VT were treated in 133 procedures. One session was sufficient in 49 cases; 2 sessions were required in 29 cases; 3 sessions in 6 cases and 4 sessions in 2 cases. A total of 480 DC shocks were delivered with energies of 160 to 300 joules. The CPK-MB levels were 37 +/- 30 i.u./l. The following complications were observed: 7 perioperative deaths, 2 cardiac tamponades requiring surgical drainage, 1 permanent AV block, 5 reversible left ventricular failures. Follow up concerned 79 patients: 5 were followed up for less than 3 months with death occurring from cardiac failure which had been present before the ablation procedure without recurrence of VT. Seventy-four patients were followed up from 3 to 111 months (56 +/- 33 months). Thirty-two patients were without any antiarrhythmic therapy and had no recurrence of VT. Two of these patients developed a different form of VT 2 and 6 years after the catheter ablation.(ABSTRACT TRUNCATED AT 250 WORDS)

[Emergency treatment of chronic ventricular tachycardia after myocardial infarction by endocavitary fulguration]. / Traitement d'urgence de la tachycardie ventriculaire chronique après infarctus du myocarde par la fulguration endocavitaire.

Endocavitary catheter ablation consists of delivering an impulse of 160 to 240 joules via a catheter used for electrophysiological investigation resulting in an electrical discharge which, in addition to its thermal effects may alter the arrhythmogenic substrate mechanically. This method was used in 2 patients with resistant and recurrent VT after myocardial infarction complicated by ventricular aneurysm. Two sessions of catheter ablation were necessary in both patients, but in the second one a single shock was successful in critically ill patients with VT. The first patient has been followed up for 20 months and the second for 13 months. Under prophylactic antiarrhythmic therapy, neither patient has had recurrence of the ventricular arrhythmias which had previously led to their hospitalisation. Catheter ablation is therefore presented as a technique which may be performed in the electro-physiological laboratory and repeated in cases of incomplete efficacy. This method may be used successfully in the treatment of chronic VT after myocardial infarction complicated by ventricular aneurysm.

[Fulguration and radiofrequency in ventricular tachycardia]. / Fulguration et radiofréquence dans la tachycardie ventriculaire.

Eighty-nine cases of ventricular tachycardia, resistant to antiarrhythmic therapy, were treated over a 10 year period by high energy D ablation (fulguration). This series included 37 cases of myocardial infarction with a mean ejection fraction of 30%. The mean follow-up period of the survivors was 61 months and clinical efficacy was 87.9%. Twenty-three cases of arrhythmogenic right ventricular dysplasia, aged 40 years, and with an ejection fraction of 57%, followed up for 71 months, had a clinical efficacy of 83%. Twelve patients had verapamil sensitive (fascicular) ventricular tachycardia. Their age was 30, their ejection fraction 65%, the follow-up period 55 months, and the clinical efficacy was 100%. Ten patients had primary dilated cardiomyopathy. Their age was 35, their ejection fraction 23%, the follow-up period of 38 months with a clinical efficacy of 80%. Four patients, aged 21, had operated congenital heart disease with an ejection fraction of 60%, a follow-up of 36 months and a clinical efficacy of 100%. Finally, 3 patients had idiopathic infundibular ventricular tachycardia. Their age was 36, the ejection fraction 62%, the follow-up period was 72 months and the clinical efficacy was 67%. Non lethal complications were observed in 16% of cases, mainly haemopericardium requiring pericardocentesis in 4.5% of cases. The operative mortality and in the month following ablation was 9.2%, observed mainly during the learning period. These results show that fulguration is not without risk, but in skilled hands, it gives remarkable results in the majority of cases.(ABSTRACT TRUNCATED AT 250 WORDS)