The high-pressure behavior of monoclinic VO_2 is revisited by a combination of Raman spectroscopy and X-ray diffraction on a single crystal under hydrostatic conditions at room temperature. A soft mode is observed up to P_c = 13.9(1) GPa. At this pressure, an isostructural phase transition between two monoclinic phases M_1 and M_1' hinders this instability. The features of this transformation (no apparent volume jump) indicate that the compression at ambient temperature passes close to a critical point. An analysis based on the Landau theory of phase transitions gives a complete description of the P-T phase diagram. The M1' is characterized by spontaneous displacements of the oxygen sub-lattice without any strong modification of the VV dimers distances nor the twist angle of vanadium chains. The spontaneous displacements of oxygen and the spontaneous deformations of the (b_M1, c_M1) plane follow the same quadratic dependence with pressure and scales with spontaneous shifts of the Raman phonons located at 225, 260 and 310 cm^-1. Pressure-induced shifts of the Raman peaks allows for new assignment of several Raman modes. In particular, the A_g(1)+B_g(1) modes at 145 cm^-1 are identified as the vanadium displacive phonons. A second transformation in the metallic phase X, which is found triclinic (P1̅) is observed starting at 32 GPa, with a wide coexistence region (up to 42 GPa). Upon decompression, phase X transforms, between 20 GPa and 3 GPa, to another phase that is neither the M_1' nor M_1 phase. The structural transitions identified under pressure match with all the previously reported electronic modifications confirming that lattice and electronic degrees of freedom are closely coupled in this correlated material.

The high-pressure behavior of monoclinic VO2 is revisited by a combination of Raman spectroscopy and x-ray diffraction on a single crystal under hydrostatic conditions at room temperature. A soft mode is observed up to Pc = 13.9(1) GPa. At this pressure, an isostructural phase transition between two monoclinic phases M1 and M1 hinders this instability. The features of this transformation (no apparent volume jump) indicate that the compression at ambient temperature passes close to a critical point. An analysis based on the Landau theory of phase transitions gives a complete description of the P -T phase diagram. The M1 is characterized by spontaneous displacements of the oxygen sublattice without any strong modification of the VV dimers distances nor the twist angle of vanadium chains. The spontaneous displacements of oxygen and the spontaneous deformations of the (bM1, cM1) plane follow the same quadratic dependence with pressure and scales with spontaneous shifts of the Raman phonons located at 225, 260, and 310 cm-1. Pressure-induced shifts of the Raman peaks allow for new assignment of several Raman modes. In particular, the Ag(1) +Bg(1) modes at 145 cm-1 are identified as the vanadium displacive phonons. A second transformation in the metallic phase X, which is found triclinic (P1 over bar ) is observed starting at 32 GPa, with a wide coexistence region (up to 42 GPa). Upon decompression, phase X transforms, between 20 and 3 GPa, to another phase that is neither the M1 norM1 phase. The structural transitions identified under pressure match with all the previously reported electronic modifications confirming that lattice and electronic degrees of freedom are closely coupled in this correlated material.

We report the evolution of nematic fluctuations in FeSe 1− x S x single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) x c ~ 0.17 via Raman scattering. The Raman spectra in the B 1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.

Layered oxides AxMnO≈2,yH2O (A = Na, K, NH4, Cs or Mn2+) have been dehydrated and intercalated by lithium. Dehydration is accompanied by a shrinking of the interlayer space height depending on the size of A. The valence of Mn is reduced to +2 by reaction with n-butyllithium. Electrochemical studies of cells AxMnO≈2-graphite | LiC1O4-PC | Li showed two main reaction peaks at ca. 3V and below 2 V. The first reaction is a reversible reduction to a disordered Mn3+ phased. Capacities at the third cycle are ca. 300 Wh/kg. No marked difference was found as a function of A or of the degree of order of the starting compound.

We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature T_{c} reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the fourfold enhancement of T_{c} at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical and electronic structure calculations using local density approximation combined with dynamical mean field theory. Our results reveal two different regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave Pomeranchuk instability of the Fermi surface at low pressure and a magnetic driven orthorhombic distortion at higher pressure.

FeSe(001) with a high defect density was studied in the superconductiong phase by means of scanning tunneling spectroscopy at low temperature. Quasiparticle excitations surrounding structural defects are found inside the superconducting gap. This spectral intensity is used for revealing additional scattering channels for the quasiparticle excitations. The related wave vectors are located around the M points of the Brillouin zone and follow C-2 symmetry. We attribute the symmetry and the observed lack of dispersion of the scattering channels to the interaction of Bogoliubov quasiparticles with nematic order.

In the spinel compound GeCo2O4, the Co2+ pyrochlore sublattice presents remarkable magnetic-field-induced behaviors that we unveil through neutron and x-ray single-crystal diffraction. The Neel-ordered magnetic phase is entered through a structural lowering of the cubic symmetry. In this phase, when a magnetic field is applied along a 2-fold cubic direction, a spin-flop transition of one-fourth of the magnetic moments releases the magnetic frustration and triggers magnetostructural effects. At high field, these ultimately lead to an unusual spin reorientation associated with structural changes.

Significance Anisotropic electron liquids are ubiquitous in many correlated electron systems. Among them, electron nematics, which break rotation but not translation symmetry, are believed to play a key role in the physics of both cuprates and iron-based superconductors (Fe SC). However the study of electron nematicity has been hampered by the lack of an adequate probe of its associated fluctuations and susceptibility, making it difficult to track its origin. Here, using polarization-resolved Raman scattering, we report the detection of critical nematic fluctuations in the charge channel in the Fe SC compound FeSe. The strong enhancement of the associated nematic susceptibility allows us to link the appearance of nematicity to a symmetry-breaking distortion of the Fermi surface.

We report the growth of high quality FeSe single crystals using chemical vapor transport based on an AlCl3/KCl eutectic and their physical properties were fully characterized by magnetic, transport and specific heat measurements. Their critical superconducting temperature at T-c similar to 8.7 K (width < 0.7 K) and structural tetragonal-orthorhombic transition at T-s similar to 85-90 K confirm the values already reported in the literature for similar crystals. In addition, the non-variation of T-s under high magnetic field (up to 14 T) determined by magnetic and specific heat measurements indicates that spin fluctuations are not directly involved in the structural transition. Finally, high temperature electrical resistivity measurements up to 600 K evidence a maximum around 350 K. The origin of this crossover from metallic behavior at low temperature to a semiconducting-like regime at high temperature is discussed and could be associated with a change of carrier density above 350 K. On the other hand, the tentative growth of Te-substituted Fe(Se1-xTex) crystals using the same growth method resulted in crystals with x(Te) < 1% and a slightly lower T-c compared to pure FeSe ones, which may be related to transition metal-chalcogen non-stoichiometry. Finally, Cr doping at the Fe site was also attempted. In contrast to previous reports in the literature, our results suggest that Cr does not substitute for Fe in the FeSe crystal but agglomerates in Cr-rich (Cr, Fe)Se-2 inclusions, and T-c of these (Fe1-xCrx) Se crystals is not increased, but slightly decreased in comparison to stoichiometric FeSe crystals.

We have synthesized for the first time the metastable compound 1T-CrTe2. We have done its complete structural characterization and measured its magnetization, specific heat and electrical resistivity between 4 and 330 K. We have also performed detailed band structure calculations. We have found that it crystallizes in the CdI2 structure type and that its electrical resistance follows a metallic behaviour below room temperature. Its magnetization and specific heat curves show that the compound has a transition to a ferromagnetic state at TC = 310 K, with the magnetic moments ordered parallel to the basal plane. From the specific heat measurements and the ferromagnetic solutions obtained from our DFT calculations, we conclude that the ferromagnetism is of itinerant nature.

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (H-c2) of the iron-based superconductor FeSe (T-c = 8.6K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55T and temperatures down to 1.6K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of Hc2 for magnetic field applied both parallel (H parallel to ab) and perpendicular (H parallel to c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for H parallel to c, a Pauli paramagnetic contribution is evidenced for H parallel to ab with Maki parameter alpha = 2.1, corresponding to Pauli field H-P = 36.5T. Copyright (C) EPLA, 2015

The effect of selenium substitution by sulphur on the structural and physical properties of antiferromagnetic TlFe1.6+delta Se-2 has been investigated via neutron, x-ray and electron diffraction, and transport measurements. The root 5a x root 5a x c super-cell related to the iron vacancy ordering found in the pure TlFe1.6Se2 selenide is also present in the S-doped TlFe1.6+delta(Se1-xSx)(2) compounds. Neutron scattering experiments show the occurrence of the same long range magnetic ordering in the whole series i.e. the 'block checkerboard' antiferromagnetic structure. In particular, this is the first detailed study where the crystal structure and the root 5a x root 5a antiferromagnetic structure is characterized by neutron powder diffraction for the pure TlFe1.6+delta S2 sulphide over a large temperature range. We demonstrate the strong correlation between occupancies of the crystallographic iron sites, the level of iron vacancy ordering and the occurrence of block antiferromagnetism in the sulphur series. Introducing S into the Se sites also increases the Fe content in TlFe1.6+delta(Se1-xSx)(2) which in turn leads to the disappearance of the Fe vacancy ordering at x = 0.5 +/- 0.15. However, by reducing the nominal Fe content, the same root 5a x root 5a x c vacancy ordering and antiferromagnetic order can be recovered also in the pure TlFe1.6+delta S2 sulphide with a simultaneous reduction in the Neel temperature from 435 K in the selenide TlFe1.75Se2 to 330 K in the sulphide TlFe1.5S2. The magnetic moment remains high at low temperature throughout the full substitution range, which contributes to the absence of superconductivity in these compounds.

This paper reports the effect of high-spin Co2+-doped CuCrO2 delafossite-type oxide on the structure and physical properties. X-ray diffraction and Raman spectroscopy show that the structure is maintained for all Co-doped samples for chromium. The incorporation of this element generates anisotropic microstrains in the structure. The temperature dependence of zero field-cooling magnetization was measured. All samples exhibit an AFM transition around 24 K. The high-spin state and the shift due to the exchange splitting of the conduction band suggest strong hybridization between carriers in the Cr 3d t(2g) band and the t(2g) states of the high-spin Co2+ to develop other spin orders benefiting to enhance magnetic susceptibility and support the evidence of new FM transition. The coupling between the magnetic order and ferroelectric order is also characterized.

In this paper, we report the effect of non-magnetic Zn-doped CuCrO2. The structure, Raman spectroscopy, magnetic properties, dielectric permittivity, and electric polarization have been investigated. The incorporation of Zn2+ generates very anisotropic microstrains in the structure. The temperature dependence of magnetic susceptibly for all samples exhibits a magnetic dilution and paramagnetic behavior at high temperature. It is argued that non-magnetic substitution destabilizes the antiferromagnetic order of Cr3+ ions and modulates the spin configuration. The coupling between the magnetic order, dielectric permittivity and polarization is also characterized.

This work describes the scandium doping effect on the structural and magnetic properties of delafossite-type oxides CuCr1 - xScxO2. The lattice parameters were found to vary according to Vegard's low. A reflection broadening is observed, that is ascribed to local lattice distortion due to the ionic radius difference between Cr3+ and the non-magnetic dopants. Magnetic susceptibility measurements show that the dominant interactions are antiferromagnetic (AFM) but that doping induces significant changes. The coupling between the local spins at the Cr sites and doped metal transition may enhance spin fluctuations at the Cr sites, which break the residual magnetic degeneracy as fluctuation-induced symmetry breaking in a highly magnetic degenerate ground state manifold of some frustrated systems.

The effect of selenium substitution by sulfur in the Tl1−yFe2−zSe2 antiferromagnet was studied by x-ray and electron diffraction, magnetization and transport measurements. Tl0.8Fe1.5(Se1−xSx)2 (nominal composition) solid solution was synthesized in the full x range (0 ≤ x(S) ≤ 1) using the sealed tube technique. No superconductivity was found down to 4.2 K in the series despite the fact that the optimal crystallographic parameters, determined by Rietveld refinements, are reached in the series (i.e. the Fe–(Se, S) interplane height and (Se, S)–Fe–(Se, S) angle for which the critical superconducting transition Tc is usually maximal in pnictides). A quasi-full Tl site (y ∼ 0.05) compared to significant alkaline deficiency (y = 0.2–0.3) in analogous A1−yFe2−zSe2 (A = K, Rb, Cs), and the resulting differences in iron valency, density of states and doping, are suggested as an explanation for this absence of superconductivity. Transmission electron microscopy confirmed the existence of an ordered iron vacancies network in the samples of the Tl0.8Fe1.5(Se1−xSx)2 series in the form of the tetragonal 5 a × 5 a × c?> superstructure (I4/m). The Néel temperature (TN) indicating the onset of antiferromagnetism order in this 5 a × 5 a × c?> supercell is found to decrease from 450 K in the selenide (x = 0) to 330 K in the sulfide (x = 1). Finally, we demonstrate a direct linear relationship between TN and the Fe–(Se, S) bond length (or Fe–(Se, S) height).