arXiv subjects

F. B. Santos

Publications and source records attributed to F. B. Santos.

8 recordsLinked to original sources

arXiv2023

Nodeless superconductivity in the noncentrosymmetric ThIrSi compound

The ThIrSi superconductor, with $T_c = 6.5$ K, is expected to show unusual features in view of its noncentrosymmetric structure and the presence of heavy elements featuring a sizable spin-orbit coupling. Here, we report a comprehensive study of its electronic properties by means of local-probe techniques: muon-spin rotation and relaxation ({\textmu}SR) and nuclear magnetic resonance (NMR). Both the superfluid density $ρ_\mathrm{sc}(T)$ (determined via transverse-field {\textmu}SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity. Furthermore, the absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field {\textmu}SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ThIrSi. Temperature-dependent upper critical fields as well as field-dependent superconducting muon-spin relaxations suggest the presence of multiple superconducting gaps in ThIrSi.

cond-mat.supr-con↗

arXiv2022

Differences in the Resistive and Thermodynamic Properties of the Single Crystalline Chiral Superconductor Candidate SrPtAs

$\require{mediawiki-texvc}$The locally non-centrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P6_{3}$/$mmc$ with lattice parameters $a$ = 4.2445(4) $Å$ and $c$ = 8.9513(18) $Å$. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at T$_c$ $\sim$2.2 K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

cond-mat.supr-con↗

arXiv2021

Spatially inhomogeneous superconductivity in UTe2

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$.

cond-mat.str-el↗

arXiv2020

Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate valence UTe2

UTe$_2$ is a recently discovered unconventional superconductor that has attracted much interest due to its many intriguing properties - a large residual density-of-states in the superconducting state, re-entrant superconductivity in high magnetic fields, and potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe$_2$ under applied pressure reveals key new insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at P$_{c1}$ = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa we detect two bulk transitions that trigger changes in the resistivity which are consistent with antiferromagnetic order, rather than ferromagnetism. The presence of both bulk magnetism and superconductivity at pressures above P$_{c2}$ = 1.4 GPa results in a significant temperature difference between resistively and thermodynamically determined transitions into the superconducting state, which indicates a suppression of the superconducting volume fraction by magnetic order. Third, the emergence of magnetism is accompanied by an increase in valence towards a U$^{4+}$ (5f2) state, which indicates that UTe$_2$ exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more significant role on the superconducting state of UTe$_2$ than previously thought.

cond-mat.str-el↗

arXiv2019

Two-band superconductivity with unconventional pairing symmetry in HfV$_2$Ga$_4$

In this letter, we have examined the superconducting ground state of the HfV$_2$Ga$_4$ compound using resistivity, magnetization, zero-field (ZF) and transverse-field (TF) muon-spin relaxation and rotation ($μ$SR) measurements. Resistivity and magnetization unveil the onset of bulk superconductivity with $T_{\bf c}\sim$ 3.9~K, while TF-$μ$SR measurements show that the temperature dependence of the superfluid density is well described by a nodal two-gap $s$+$d$-wave order parameter model. In addition, ZF muon relaxation rate increases with decreasing temperature below 4.6 K, indicating the presence of weak spin fluctuations. These observations suggest an unconventional multiband nature of the superconductivity possibly arising from the distinct $d$-bands of V and Hf ions with spin fluctuations playing an important role. To better understand these findings, we carry out first-principles electronic-structure calculations, further highlighting that the Fermi surface consists of multiple disconnected sheets with very different orbital weights and spin-orbit coupling, bridging the way for a nodal multiband superconductivity scenario. In this vein, therefore, HfV$_2$Ga$_4$-family stands out as an open avenue to novel unexplored unconventional superconducting compounds, such as ScV$_2$Ga$_4$ and ZrV$_2$Ga$_4$, and other many rare earths based materials.

cond-mat.supr-con↗

arXiv2019

Strong Electronic Interaction and Signatures of Nodal Superconductivity in Zr$_5$Pt$_3$C$_x$

The physical properties of the Zr$_5$Pt$_3$ compound with interstitial carbon in hexagonal D8$_8$-structure was investigated. A set of macroscopic measurements reveal a bulk superconducting at approximately 7 K for Zr$_5$Pt$_3$C$_{0.3}$ close to Zr$_5$Pt$_3$, also with a correlate anomalous resistivity behavior. However, both the signatures of strong electron-electron interaction, and the electronic contribution to specific heat, increase dramatically with the C doping. For the first time the x-ray photoelectron spectra compared with DFT/PWLO calculations of electronic structure show a complex Fermi surface with high density of states for Zr$_5$Pt$_3$. Also results show the signature of unconventional superconductivity. Indeed, was observed an unusual behavior for lower and upper critical field diagrams of Zr$_5$Pt$_3$C$_{0.3}$. The temperature dependence of penetration length and electronic contribution to specific heat suggests that electronic pairing deviates of $s$-wave the BCS scenario.

cond-mat.supr-con↗

arXiv2018

Elastic anisotropy and thermal properties of extended linear chain compounds MV$_2$Ga$_4$ (M = Sc, Zr, Hf) from ab-initio calculations

MV$_2$Ga$_4$ (M = Sc, Zr, Hf) compounds belong to an emerging class of materials showing a unique combination of unusual superconducting behavior with extended linear chains in the crystal structure. In order to gain insights {into} its mechanical and thermal properties, we have performed first-principles electronic-structure calculations in the framework of the Density Functional Theory (DFT). From the calculated second-order elastic constants, we have systematically shown that the extended linear vanadium chain substructures indeed give rise to an anisotropic regime in the elastic and mechanical moduli. The high density of valence and conduction electrons along the linear vanadium chains leads to a directional dependence of the reciprocal linear compressibility, Young's modulus and shear modulus. Poisson's ratio for several elongation directions is also drastically affected by the presence of extended V chains. If the elongation is along the V chains, all compounds exhibit {practically} the same Poisson ratio in directions perpendicular to it, further highlighting the importance of the V chains to the mechanical properties. Moreover, based on our results, we have discussed the possible consequences of the elastic anisotropy on the superconducting properties of the compounds. Finally, using the Debye-Grüneisen approximation, our calculations of thermal properties show {a good agreement with the available experimental low temperature heat capacity data above the superconducting critical temperature.

cond-mat.mtrl-sci↗

arXiv2018

Insights on unconventional superconductivity in HfV$_2$Ga$_4$ and ScV$_2$Ga$_4$ from first principles electronic structure calculations

The HfV$_2$Ga$_4$ compound was recently reported to exhibit unusual bulk superconducting properties, with the possibility of multiband behavior. To gain insight into its properties, we performed ab-initio electronic structure calculations based on the Density Functional Theory (DFT). Our results show that the density of states at the Fermi energy is mainly composed by V--$d$ states. The McMillan formula predicts a superconducting critical temperature ($T_{c}$) of approximately $3.9\,$K, in excellent agreement with the experimental value at $4.1\,$K, indicating that superconductivity in this new compound may be explained by the electron-phonon mechanism. Calculated valence charge density maps clearly show directional bonding between Hf and V atoms with 1D highly populated V-chains, and some ionic character between Hf--Ga and V--Ga bonds. Finally, we have shown that there are electrons occupying two distinct bands at the Fermi level, with different characters, which supports experimental indications of possible multiband superconductivity. Based on the results, we propose the study of a related compound, ScV$_2$Ga$_4$, showing that it has similar electronic properties, but probably with a higher $T_c$ than HfV$_2$Ga$_4$.

cond-mat.supr-con↗