arXiv subjects

P. Fertey

Publications and source records attributed to P. Fertey.

8 recordsLinked to original sources

arXiv2026

Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La$_2$SmNi$_2$O$_7$

We investigated the structural properties of bilayer nickelate La$_2$SmNi$_2$O$_7$ as a function of pressure and temperature. At ambient conditions, we show that the material crystallizes as a monoclinic superstructure distinct from the one previously reported and close to the pseudo-orthorhombic structure of pristine La$_3$Ni$_2$O$_7$. No signatures of satellite reflections associated with charge density wave (CDW) ordering are detected at low temperature. Upon compression, a sequence of pressure-induced structural transitions from monoclinic to orthorhombic 15 GPa and then tetragonal 21 GPa symmetry is observed. Within the superconducting dome, the quality of the X-ray diffraction data enables structural refinements enabling theoretical models to understand the emergence of superconductivity.

cond-mat.str-el↗

arXiv2026

BaFe2Se3 a quasi-unidimensional non-centrosymmetric superconductor

The spin-ladder compounds of the BaFe2X3 (X = chalcogen) family may be viewed as dimensional reductions (along stripe-like motifs) of the two-dimensional iron-based pnictide planes extensively studied since 2006. Remarkably, despite their reduced dimensionality, these materials retain the capacity for unconventional ground states, exemplified by the emergence of superconductivity in \bfse\ under applied pressure beyond 10 GPa, following a structural phase transition at 4 GPa. Here, we report a comprehensive investigation combining high-resolution single-crystal X-ray diffraction, infrared spectroscopy, and ab initio calculations, which together elucidate the true crystallographic nature of this pressure-induced superconducting phase. While X-ray diffraction alone reveals a symmetry lowering from the widely accepted orthorhombic Cmcm group to a monoclinic structure, it lacks sufficient sensitivity to resolve the precise space group. By integrating vibrational spectroscopy with density functional theory, we provide unambiguous evidence that the high-pressure phase is non-centrosymmetric, adopting the polar space group P2_1. These findings not only revise the structural assignment of \bfse\ in its superconducting state but also establish its non-centrosymmetric character (an essential ingredient for potential unconventional pairing mechanisms- thereby opening new perspectives on the interplay between lattice symmetry, dimensionality, and superconductivity in iron-based materials.

cond-mat.supr-con↗

arXiv2025

New insight on the phase diagram of the superconducting iron spin ladder BaFe$_2$S$_3$

BaFe$_2$S$_3$ and BaFe$_2$Se$_3$ are the only two quasi-one-dimensional iron-based compounds that become superconductors under pressure. Interestingly, these two compounds exhibit different symmetries and properties. While more detailed and recent studies on BaFe$_2$Se$_3$ using single crystals have advanced the filed towards a more universal description of this family, such a study is still lacking for the compound BaFe$_2$S$_3$. Here, we present a detailed study of the crystalline and magnetic structure performed on single crystals using X-ray and neutron diffraction. We demonstrate a polar structure at room temperature within the $Cm2m$ space group, followed by a structural transition at 130 K to the polar $Pb2_1m$ space group. This space group remains unchanged across the magnetic transition at $T_N =95$ K, revealing multiferroic characteristics with a weak magnetoelastic coupling. The determined magnetic structure is monoclinic ($P_am$), with non-collinear magnetic moments, displaying a significant angle of 18$^\circ$ relative to the $a$-axis in the $(a, c)$ plane. This reexamination of the temperature-dependent properties of BaFe$_2$S$_3$ provides new insights into the physics of this system from multiple key perspectives.

cond-mat.str-el↗

arXiv2024

Giant Strain Tunability in Polycrystalline Ceramic Films via Helium Implantation

Strain engineering is a powerful tool routinely used to control and enhance properties such as ferroelectricity, magnetic ordering, or metal-insulator transitions. Epitaxial strain in thin films allows manipulation of in-plane lattice parameters, achieving strain values generally up to 4%, and above in some specific cases. In polycrystalline films, which are more suitable for functional applications due to their lower fabrication costs, strains above 1% often cause cracking. This poses challenges for functional property tuning by strain engineering. Helium implantation has been shown to induce negative pressure through interstitial implantation, which increases the unit cell volume and allows for continuous strain tuning with the implanted dose in epitaxial monocrystalline films. However, there have been no studies on the transferability of helium implantation as a strain-engineering technique to polycrystalline films. Here, we demonstrate the technique's applicability for strain engineering beyond epitaxial monocrystalline samples. Helium implantation can trigger an unprecedented lattice parameter expansion of up to 3.2% in polycrystalline BiFeO3 films without causing structural cracks. The film maintains stable ferroelectric properties with doses up to 1E15 He/cm2. This finding underscores the potential of helium implantation in strain engineering polycrystalline materials, enabling cost-effective and versatile applications.

cond-mat.mtrl-sci↗

arXiv2021

Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally by helium implantation. Our combined Raman, XRD and TEM study shows that the implantation causes an elongation of the BiFeO3 unit cell and ultimately a transition towards the so-called super-tetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like BiFeO3 appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

cond-mat.mtrl-sci↗

arXiv2016

Pressure-induced phonon freezing in the ZnSeS II-VI mixed crystal: phonon-polaritons and ab initio calculations

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the phonon-polariton transverse optical modes (with mixed electrical and mechanical character) of the II-VI ZnSeS mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly resolved percolation-type Zn-S Raman doublet of the three oscillator [1(Zn-Se),2(Zn-S)] ZnSe68S32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende-to-rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe68S32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn- S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar phonon freezing was earlier evidenced with the well-resolved percolation-type Be-Se doublet of ZnBeSe [Pradhan et al. Phys. Rev. B 81, 115207 (2010)], that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse and convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.

cond-mat.mtrl-sci↗

arXiv2010

Flux pinning in (1111) iron-pnictide superconducting crystals

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5% - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)-plane is consistent with an order-disorder transition of the vortex lattice.

cond-mat.supr-con↗

arXiv1999

Microwave transport approach to the coherence of interchain hopping in (TMTSF)2PF6

We report a microwave study of the longitudinal and transverse transport properties of the quasi-one-dimensional organic conductor (TMTSF)$_2$PF$_6$ in its normal phase. The contactless technique have provided a direct measurement of the temperature profile of the resistivity along the {\bf b'} direction and in magnetic fields up to 14 T. A characteristic energy scale ($T_x \sim 40 K$) has been observed which delimits a transient regime from an insulating to a metallic behavior. This anomalous profile is discussed in terms of the onset of coherent transport properties along the {\bf b'} direction below 40 K. This is also supported by the observation of a finite longitudinal and transverse magnetoresitances only below 40 K, indicative of a two-dimensional regime. Below $T_x$, however, strong deviations with respect to a Fermi liquid behavior are evidenced.

cond-mat.str-el↗