arXiv subjects

M. N. Gastiasoro

Publications and source records attributed to M. N. Gastiasoro.

7 recordsLinked to original sources

arXiv2026

Spin-chiral electron-phonon coupling in metallic strontium titanate

Electron-phonon coupling (EPC) - the interaction between conduction electrons and quantized atomic vibrations - plays a central role in condensed matter physics and determines some of the most important properties of materials, such as electrical resistivity and superconductivity. Conventionally, EPC is assumed to be induced by the ionic electrostatic background, and electronic spin plays no role in the process. In stark contrast with this view, here we uncover a direct spin-mediated coupling mechanism between electrons and transverse polar phonons in a metal. Using far-infrared light absorption measurements of the model system SrTiO$_3$ in a magnetic field, we observe a strong spin-mediated EPC that is quantitatively consistent with recent theoretical predictions, and that generates chiral phonon modes with large effective magnetic moments. The extracted coupling strength is in good agreement with ab initio estimates and sufficiently high to explain superconductivity in SrTiO$_3$, thereby resolving a long-standing conundrum. Spin-chiral EPC should generically appear in all metals with polar phonons, and the present work could be of relevance to spintronics applications and to uncovering the origins of superconductivity in layered materials, metals with Dirac points in their electronic dispersions, and nearly ferroelectric superconductors.

cond-mat.str-el↗

arXiv2024

Electronic spin susceptibility in metallic strontium titanate

Metallic strontium titanate (SrTiO$_3$) is known to have both normal-state and superconducting properties that vary strongly over a wide range of charge carrier densities. This indicates the importance of nonlinear dynamics, and has hindered the development of a clear qualitative description of the observed behaviour. A major challenge is to understand how the charge carriers themselves evolve with doping and temperature, with possible polaronic effects and evidence of an effective mass that strongly increases with temperature. Here we use $^{47,49}$Ti nuclear magnetic resonance (NMR) to perform a comprehensive study of the electronic spin susceptibility in the dilute metallic state of strontium titanate across the doping-temperature phase diagram. We find a temperature-dependent Knight shift that can be quantitatively understood within a non-degenerate Fermi gas model that fully takes into account the complex band structure of SrTiO$_3$. Our data are consistent with a temperature-independent effective mass, and we show that the behavior of the spin susceptibility is universal in a wide range of temperatures and carrier concentrations. These results provide a microscopic foundation for the understanding of the properties of the unconventional low-density metallic state in strontium titanate and related materials.

cond-mat.str-el↗

arXiv2019

Low Temperature Specific Heat of Doped SrTiO$_3$: Doping Dependence of the Effective Mass and Kadowaki-Woods Scaling Violation

We report wide-doping-range ($8 \times 10^{17}$ to $4 \times 10^{20}$ cm$^{-3}$ Hall electron density) low temperature specific heat measurements on single crystal SrTiO$_3$:Nb, correlated with electronic transport data and tight-binding modeling. Lattice dynamic contributions to specific heat are shown to be well understood, albeit with unusual sensitivity to doping, likely related to the behavior of soft modes. Electronic contributions to specific heat provide effective masses that increase substantially, from $1.8$ to $4.8 m_e$, across the two SrTiO$_3$ Lifshitz transitions. It is shown that this behavior can be quantitatively reconciled with quantum oscillation data and calculated band structure, establishing a remarkably doping-independent mass enhancement factor of $2.0$. Most importantly, with the doping-dependent $T^2$ resistivity prefactor and Sommerfeld coefficient known, Kadowaki-Woods scaling has been tested over the entire doping range probed. Despite classic Fermi liquid behavior in electronic specific heat, standard Kadowaki-Woods scaling is dramatically violated, highlighting the need for new theoretical descriptions of $T^2$ resistivity in SrTiO$_3$.

cond-mat.str-el↗

arXiv2017

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

$^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with Mössbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed magnetic order, dominated by $Q_1=(π,0)$ and $Q_2=(0,π)$ ordering vectors. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.

cond-mat.supr-con↗

arXiv2016

Role of magnetic dopants in the phase diagram of Sm1111 pnictides: The Mn case

The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muon-spin spectroscopy (muSR), both in zero- and in transverse fields, respectively, to probe the magnetic and the superconducting order. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors [M. Gastiasoro et al., ArXiv 1606.09495], which includes correlation-enhanced RKKY-couplings between the impurity moments.

cond-mat.supr-con↗

arXiv2014

Spin excitations in the nematic phase and the metallic stripe spin-density wave phase of iron pnictides

We present a general study of the magnetic excitations within a weak-coupling five-orbital model relevant to itinerant iron pnictides. As a function of enhanced electronic correlations, the spin excitations in the symmetry broken spin-density wave phase evolve from broad low-energy modes in the limit of weak interactions to sharply dispersing spin wave prevailing to higher energies at larger interaction strengths. We show how the resulting spin response at high energies depends qualitatively on the magnitude of the interactions. We also calculate the magnetic excitations in the nematic phase by including an orbital splitting, and find a pronounced C_2 symmetric excitation spectrum right above the transition to long-range magnetic order. Finally, we discuss the C_2 versus C_4 symmetry of the spin excitations as a function of energy for both the nematic and the spin-density wave phase.

cond-mat.supr-con↗

arXiv2013

Low-energy bound states at interfaces between superconducting and block antiferromagnet regions in KxFe{2-y}Se2

The high-Tc alkali doped iron selenide superconductors KxFe{2-y}Se2 have been recently shown to be intrinsically phase separated into Fe vacancy ordered block antiferromagnetic regions and superconducting regions at low temperatures. In this work, we use a microscopic five orbital Hubbard model to obtain the electronic low-energy states near the interfaces between block antiferromagnets and superconductors. It is found that abundant low-energy in-gap bound states exist near such interfaces irrespective of whether the superconductor has d- or s-wave pairing symmetry. By contrast, it is shown how nonmagnetic scattering planes can provide a natural means to distinguish between these two leading pairing instabilities of the KxFe{2-y}Se2 materials.

cond-mat.supr-con↗