arXiv subjects

A. Sulpice

Publications and source records attributed to A. Sulpice.

15 recordsLinked to original sources

arXiv2026

Stabilization of a non-superconducting, orthorhombic phase by over-hydrogenating LaFeSiH

Chemical composition provides a powerful route to tune the electronic ground state of iron-based superconductors and other quantum materials, yet access to highly doped phases remains limited. Here we demonstrate that high-pressure thermal decomposition of hydrogen-rich precursors enables over-hydrogenation of LaFeSi. Using anthracene, we synthesize tetragonal superconducting LaFeSiH, including a single hydrogen site, while ammonia borane yields a structurally distorted over-hydrogenated phase, LaFeSiH1+x, with an orthorhombic structure. Chemical analysis reveal excess hydrogen (x ~ 0.6), implying a second H site in LaFeSiH1.6 whose localization and occupancy are determined by neutron diffraction. In contrast to metallic LaFeSi and superconducting LaFeSiH, orthorhombic LaFeSiH1.6 exhibits semiconductor-like behavior. Upon hydrogen release near 100 {\deg}C, it transforms into tetragonal superconducting LaFeSiH1+{\delta} ({\delta} << 0.6). These results establish the chemical flexibility of the layered LaFeSiX (X = H, O, F) family and provide access to a high hydrogen-doping regime, creating new opportunities to investigate superconductivity in Fe-based silicides.

cond-mat.supr-con↗

arXiv2023

Magnetic and structural properties of the iron silicide superconductor LaFeSiH

The magnetic and structural properties of the recently discovered pnictogen/chalcogen-free superconductor LaFeSiH ($T_c\simeq10$~K) have been investigated by $^{57}$Fe synchrotron M{\"o}ssbauer source (SMS) spectroscopy, x-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance spectroscopy (NMR). No sign of long range magnetic order or local moments has been detected in any of the measurements and LaFeSiH remains tetragonal down to 2 K. The activated temperature dependence of both the NMR Knight shift and the relaxation rate $1/T_1$ is analogous to that observed in strongly overdoped Fe-based superconductors. These results, together with the temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous metal, far from any magnetic or nematic instability, and with similar Fermi surface properties as strongly overdoped iron pnictides. This raises the prospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier concentration through appropriate chemical substitutions. Additional SMS spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no measurable hyperfine field.

cond-mat.supr-con↗

arXiv2022

Superconductivity in the crystallogenide LaFeSiO$_{1-\delta}$ with squeezed FeSi layers

Pnictogens and chalcogens are both viable anions for promoting Fe-based superconductivity and intense research activity in the related families has established systematic correlation between the Fe-anion height and the superconducting critical temperature $T_c$, with an optimum Fe-anion height of $\sim$ 1.38 \r{A}. Here, we report the discovery of superconductivity in a novel compound LaFeSiO$_{1-\delta}$ that incorporates a crystallogen element, Si, and challenges the above picture: considering the strongly squeezed Fe-Si height of 0.94 \r{A}, the superconducting transition at $T_{c}$ = 10 K is unusually high. In the normal state, the resistivity displays non-Fermi-liquid behavior while NMR experiments evidence weak antiferromagnetic fluctuations. According to first-principles calculations, the Fermi surface of this material is dominated by hole pockets without nesting properties, which explains the strongly suppressed tendency towards magnetic order and suggests that the emergence of superconductivity materializes in a distinct set-up, as compared to the standard $s_\pm$- and $d$-wave electron-pocket-based situations. These properties and its simple-to-implement synthesis make LaFeSiO$_{1-\delta}$ a particularly promising platform to study the interplay between structure, electron correlations and superconductivity.

cond-mat.supr-con↗

arXiv2017

Iron-based superconductivity extended to the novel silicide LaFeSiH

We report the synthesis and characterization of the novel silicide LaFeSiH displaying superconductivity with onset at 11 K. We find that this pnictogen-free compound is isostructural to LaFeAsO, with a similar low-temperature tetragonal to orthorhombic distortion. Using density functional theory we show that this system is also a multiband metal in which the orthorhombic distortion is likely related to single-stripe antiferromagnetic order. Electrical resistivity and magnetic susceptibility measurements reveal that these features occur side-by-side with superconductivity, which is suppressed by external pressure.

cond-mat.supr-con↗

arXiv2012

Anomalous dependence of the c-axis polarized Fe B$_{1g}$ phonon mode with Fe and Se concentrations in Fe$_{1+y}$Te$_{1-x}$Se$_x$

We report an investigation of the lattice dynamical properties in a range of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ compounds, with special emphasis on the c-axis polarized vibration of Fe with B$_{1g}$ symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se $x$ and excess Fe $y$ concentrations. In parent compound Fe$_{1+y}$Te, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature $T_N$. The situation smoothly evolves towards a regular anharmonic behavior as Te is substituted for Se and long range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B$_{1g}$ phonon at low temperatures. We performed Density Functional Theory (DFT) ab-initio calculations within the local density approximation (LDA) to calcuate the phonon frequencies including magnetic polarization and Fe non-stoichiometry in the Virtual Crystal Approximation (VCA). We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low energy magnetic fluctuations that can not be treated accurately within these approaches. As recently revealed by neutron scattering and $μ$-SR studies, these phenomena occur in the temperature range where anomalous decay of the B$_{1g}$ phonon is observed, and suggests a peculiar coupling of this mode with local moments and spin fluctuations in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$.

cond-mat.supr-con↗

arXiv2011

Effect of the isoelectronic substitution of Sb for As on the magnetic and structural properties of LaFe(As1-xSbx)O

The antiferromagnetic order and structural distortion in the LaFe(As1-xSbx)O system have been investigated by powder neutron diffraction and physical properties measurements. Polycrystalline samples of LaFe(As1-xSbx)O (x<0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe magnetic moment is observed.

cond-mat.supr-con↗

arXiv2011

Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsO$_{1-x}$F$_{x}$

The electrical resistivity, crystalline structure and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO$_{0.81}$F$_{0.19}$ have been studied up to pressures ~20GPa. The correlation between the pressure dependence of the superconducting transition temperature (Tc) and crystallographic parameters on the same sample shows clearly that a regular FeAs$_{4}$ tetrahedron maximizes Tc, through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.

cond-mat.supr-con↗

arXiv2010

Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals up to 28 Tesla

We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c \sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H\|c$ and $H\|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {\it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T \rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c \sim 0.99$ (for $H\|ab$) which is the consequence of a very small value of the coherence length $ξ_c(0) \sim 4 Å$ (and $ξ_{ab}(0) \sim 15 Å$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $λ_{ab}(0) = 430 \pm 50$ nm and $λ_c(0) = 1600 \pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($\sim 4$), being close to the anisotropy of $H_{c2}$ for $T\rightarrow T_c$. The temperature dependence of both $λ$ ($\propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.

cond-mat.supr-con↗

arXiv2009

High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe

We have studied the structural and superconducting properties of tetragonal FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller than the rest of Fe based superconductors. At 12GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high pressure orthorhombic phase has a higher Tc reaching 34K at 22GPa.

cond-mat.supr-con↗

arXiv2006

Experimental study of the competition between Kondo and RKKY interactions for Mn spins in a model alloy system

The quasicrystal Al-Pd-Mn is a model system for an experimental study of the competition between Ruderman-Kittel-Kasuya-Yoshida (RKKY) and Kondo interactions. First, specific of such alloys, only a few Mn atoms carry an effective spin and their concentration x is tunable over several orders of magnitude, even though the Mn amount is almost constant. Second, the characteristic energy scales for the interactions lie in the Kelvin range. Hence we could study the magnetization on both side of these energy scales, covering a range of temperatures [0.1-100 K] and magnetic fields (mu_B H/k_B= 0 to 5 K) for 22 samples and x varying over 2 decades. Using very general Kondo physics arguments, and thus carrying out the data analysis with no preconceived model, we found a very robust and simple result: The magnetization is a sum of a pure Kondo (T_K=3.35K) and a pure RKKY contributions, whatever the moment concentration is and this surprisingly up to the concentration where the RKKY couplings dominate fully and thus cannot be considered as a perturbation.

cond-mat.str-el↗

arXiv2004

Decoupling of orbital and spin degrees of freedom in Li1-xNaxNiO2

In the Li1-xNaxNiO2 solid solutions three different single phase regions exist: for x > 0.9, forx = 0.7 and for x < 0.3. Although the intermediate compound does not show the cooperative Jahn-Teller transition of NaNiO2, its magnetic properties remain very similar with, in particular, the low temperature 3D magnetic ordering. Therefore, the strong coupling between orbital and spindegrees of freedom, characteristic of other oxides like perovskites, and usually invoked to explainthe absence of both long-range orbital and magnetic ordering in LiNiO2, seems not to take placein these layered compounds with 90-degree bonds. We also discuss the relevance of the O crystal fieldsplitting induced by the trigonal distortion, in generating AFM Ni-Ni in-plane interactions.

cond-mat.str-el↗

arXiv2002

Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)Bi03

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K,Ba)BiO$_3$ single crystals. The temperature dependence of the magnetic field $H_{Cp}$ corresponding to the onset of the specific heat anomaly presents a clear positive curvature. $H_{Cp}$ is significantly smaller than the field $H_Δ$ for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization present a strong deviation from the Ginzburg--Landau theory emphasazing the peculiar nature of the superconducting transition in this material.

cond-mat.supr-con↗

arXiv2001

Spin-Peierls transition with strong structural fluctuations in the vanadium oxide VOSb$_{2}$O$_{4}$

We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium oxide VOSb$_{2}$O$_{4}$, a quasi-one dimensional S=1/2 Heisenberg system. We show that the susceptibility vanishes at zero temperature, as in a gapped system, and we argue that this is due to a spin-Peierls transition with strong structural fluctuations.

cond-mat.str-el↗

arXiv1999

Biquadratic interactions and spin-Peierls transition in the spin 1 chain LiVGe2O6

The magnetic susceptibility of a new one-dimensional, S=1 system, the vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is consistent with a gapless spectrum above this temperature. We propose that this behaviour is due to the presence of a significant biquadratic exchange interaction, a suggestion supported by quantum chemistry calculations that take into account the quasi-degeneracy of the t2g levels.

cond-mat.str-el↗

arXiv1999

Enhancement of Andreev reflection by spin polarization in out-of-equilibrium ferromagnet-superconductor junctions

We report on transport measurements on Nb/Al/Gd/Al/Nb junctions.Bulk Gadolinium is a weakly polarized ferromagnet (5-7%), and is present in the junction in granular form (superparamagnet). We show that Andreev reflection is strongly enhanced by a weak polarization, obtained by applying an external magnetic field. A new model is proposed that accounts for this effect. The transport is described in terms of ``hot'' carriers that experience Zeeman splitting due to the spin polarized background. The Landauer formula is $G=(4e^2)/h A(h_b)$, $h_b$ being the exchange field at the grain boundary, and A(E) the Andreev current coefficient, that increases with E for resistive junctions.

cond-mat.supr-con↗