arXiv subjects

Marcos A. Avila

Publications and source records attributed to Marcos A. Avila.

11 recordsLinked to original sources

arXiv2026

Enhanced Kadowaki-Woods Ratio and Weak-Coupling Superconductivity in Noncentrosymmetric YPt$_2$Si$_2$ Single Crystals

Superconductivity in noncentrosymmetric RPt2Si2 (R = rare earth) compounds exhibit a rich playground to explore the competition between different ground states, such as unconventional superconductivity, antiferromagnetism and charge density wave. Here, we report the successful single crystal synthesis of noncentrosymmetric YPt2Si2 superconductor, with a transition temperature Tc = 1.67 K, via Sn flux method. The high quality of the prepared single crystals was confirmed using powder and Laue XRD measurements. The superconducting and normal state properties are investigated using electrical transport and heat capacity measurements down to 0.5 K. In the normal state, unlike LaPt2Si2, no charge density wave transition is observed in YPt2Si2, as evidenced by electrical transport and specific heat measurements. A relatively large Kadowaki-Woods ratio and a linear temperature variation of the electrical resistivity in an extended temperature range of 50-300 K suggest an unconventional normal-state in YPt2Si2. The estimated superconducting parameters indicate that YPt2Si2 is a type-II superconductor with weak electron-phonon coupling. The temperature dependence of specific heat in the superconducting state can be explained reasonably well using an isotropic two-gap model. A positive curvature near Tc in the temperature variation of upper critical field also supports the two-gap superconductivity. First-principles DFT calculations suggest a BCS-like superconducting state driven primarily by d-electron contributions. The calculated electron-phonon coupling constant identifies the material as a weak-coupling superconductor, with the McMillan-Allen-Dynes formula yielding a Tc of 1.8 K. Additionally, we provide a comparative analysis of the superconducting and normal-state properties of YPt2Si2 and compositionally similar LaPt2Si2.

cond-mat.supr-con↗

arXiv2026

Anomalous temperature dependence of the electrical resistivity in R$_3$Co$_4$Ge$_{13}$ (R = Y, Lu) single crystals

The presence of strong disorder can significantly impact electrical conduction in metallic systems. Here, we investigate the temperature dependence of the electrical resistivity, $ρ(T)$, in nonmagnetic single crystals of the Remeika-phase cage compounds R$_3$Co$_4$Ge$_{13}$ (R = Y, Lu). Contrary to the density of states (DOS) calculations in the literature, the experimentally measured $ρ(T)$ in both compounds exhibits semiconducting-like behavior, which we attribute to the strong structural disorder due to its unique crystal structure and low carrier-density. A detailed analysis of the electrical resistivity data reveals that neither the Arrhenius thermal activation law nor variable-range hopping (VRH) models can adequately describe their temperature dependence over the broad temperature range of 2-350 K. However, a model incorporating parallel conduction through both semiconducting and metallic channels provides an adequate explanation. In addition to a dominant metallic conduction below $\sim 10$~K, a negative temperature coefficient of the electrical resistivity ($dρ/dT$) is found in both samples. In the absence of magnetic impurities, the observed $dρ/dT < 0$ is interpreted in terms of the structural Kondo mechanism.

cond-mat.mtrl-sci↗

arXiv2025

Synthesis and anisotropic magnetism of singlecrystalline GdPt2Si2

Single crystals of GdPt$_2$Si$_2$ were grown using the Sn flux method, crystallizing in the CaBe$_2$Ge$_2$-type tetragonal structure with space group $P4/nmm$. Electrical resistivity, specific heat, and magnetization data revealed the presence of a double magnetic transition with $T_N \approx 8.4$~K and $T_0 \approx 6.8$~K. Analysis of the specific heat data suggest amplitude-modulated and equal-moment antiferromagnetic orderings, respectively. Field-induced magnetization and magnetic susceptibility data show a metamagnetic transition in the $H \parallel a$ direction at 2~K, as well as the suppression of the magnetic transition located at $T_0$ with increasing external magnetic field. Electron Spin Resonance (ESR) shows the Gd$^{3+}$ resonance followed by a small second resonance. Peak-to-peak linewidth ($ΔH_{pp}$) analysis reveals slight broadening at $T \sim 120$~K, indicating an increase in magnetic fluctuations at high temperatures. Ferromagnetic (FM) local polarization at high temperatures is also observed through the $g$-factor analysis, which shows a notable positive shift ($Δg$). Our results establish the fundamental physical properties of this material to aid in further understanding of the magnetism in the RPt$_2$Si$_2$ series and related non-centrosymmetric systems.

cond-mat.str-el↗

arXiv2023

Bulk conducting states of intrinsically doped Bi$_2$Se$_3$

With a large band gap and a single Dirac cone responsible for the topological surface states, Bi2Se3 is widely regarded as a prototypical 3D topological insulator. Further applications of the bulk material has, however, been hindered by inherent structural defects that donate electrons and make the bulk conductive. Consequently, controlling these defects is of great interest for future technological applications, and while past literature has focused on adding external doping elements to the mixture, a complete study on undoped Bi2Se3 was still lacking. In this work, we use the self-flux method to obtain high-quality Bi2Se3 single-crystals in the entire concentration range available on the phase-diagram for the technique. By combining basic structural characterization with measurements of the resistivity, Hall effect and Shubnikov-de Haas (SdH) quantum oscillations, the effects of these impurities on the bulk transport are investigated in samples with electron densities ranging from 10^17 cm^-3 to 10^19 cm^-3, from Se-rich to Bi-rich mixtures, respectively, evidencing the transition into a degenerate semiconductor regime. We find that electron-donor impurities, likely Se vacancies, unavoidably shift the Fermi level up to 200 meV inside the conduction band. Other impurities, like interstitial Bi and Se, are shown to play a significant role as scattering centres, specially at low temperatures and in the decoherence of the SdH oscillations. Previous open questions on Bi2Se3, such as the upturn in resistivity below 30 K, the different scattering times in transport and quantum oscillations, and the presence of additional low mobility bands, are addressed. The results outlined here provide a concise picture on the bulk conducting states in flux-grown Bi2Se3 single crystals, enabling better control of the structural defects and electronic properties.

cond-mat.mtrl-sci↗

arXiv2017

Transitions from a Kondo-like diamagnetic insulator into a modulated ferromagnetic metal in $\bm{\mathrm{FeGa}_{3-y}\mathrm{Ge}_y}$

One initial and essential question of magnetism is whether the magnetic properties of a material are governed by localized moments or itinerant electrons. Here we expose the case for the weakly ferromagnetic system FeGa$_{3-y}$Ge$_y$ wherein these two opposite models are reconciled, such that the magnetic susceptibility is quantitatively explained by taking into account the effects of spin-spin correlation. With the electron doping introduced by Ge substitution, the diamagnetic insulating parent compound FeGa$_3$ becomes a paramagnetic metal as early as at $ y=0.01 $, and turns into a weakly ferromagnetic metal around the quantum critical point $ y=0.15 $. Within the ferromagnetic regime of FeGa$_{3-y}$Ge$_y$, the magnetic properties are of a weakly itinerant ferromagnetic nature, located in the intermediate regime between the localized and the itinerant dominance. Our analysis implies a potential universality for all itinerant-electron ferromagnets.

cond-mat.str-el↗

arXiv2016

Superconducting properties and hydrostatic pressure effect on ABi3 (A=Ba,Sr) single crystals

We report on the crystal growth and characterization of ABi3 (A=Ba,Sr) superconductors. Single crystals of both compounds were grown by the self-flux technique. BaBi3 crystallized in a tetragonal structure with space group P4/mmm and SrBi3 in a cubic structure with space group Pm-3m. Superconductivity at Tc = 6.0 K for BaBi3 and Tc = 5.6 K for SrBi3 have been confirmed through dc magnetic susceptibility and electrical transport measurements. The dc magnetic susceptibility under hydrostatic pressure shows a positive pressure coefficient of dTc/dP = 1.22 K/GPa for BaBi3 and a negative pressure coefficient of dTc/dP = -0.48 K/GPa for SrBi3. The normal state electrical resistivity shows that both compounds are highly metallic in nature. The upper critical fields Hc2 evaluated by resistivity under magnetic fields $ρ(T,H)$ are 22 kOe for BaBi3 and 2.9 kOe for SrBi3. A specific heat jump of $ΔCe/γTc = 1.05$ suggests weak coupling superconductivity in BaBi3, whereas $ΔCe/γTc = 2.08$ for SrBi3 is higher than the BCS theory value of 1.43, indicating a strong coupling superconductor.

cond-mat.supr-con↗

arXiv2016

Tuning the electronic hybridization in the heavy fermion cage compound YbFe$_{2}$Zn$_{20}$ with Cd-doping

Tuning of the electronic properties of heavy fermion compounds by chemical substitutions provides excellent opportunities to further understand the physics of hybridized ions in crystal lattices. Here we present an investigation on the effects of Cd doping in flux-grown single crystals of the complex intermetallic cage compound YbFe$_{2}$Zn$_{20}$, that has been described as a heavy fermion with Sommerfeld coefficient of 535 mJ/mol.K$^{2}$. Substitution of Cd for Zn disturbs the system by expanding the unit cell and, in this case, the size of the Zn cages that surround Yb and Fe. With increasing amount of Cd, the hybridization between Yb $4f$ electrons and the conduction electrons is weakened, as evidenced by a decrease in the Sommerfeld coefficient, which should be accompanied by a valence shift of the Yb$^{3+}$ due to the negative chemical pressure effect. This scenario is also supported by the low temperature dc-magnetic susceptibility, that is gradually suppressed and evidences an increment of the Kondo temperature, based on a shift to higher temperatures of the characteristic broad susceptibility peak. Furthermore, the DC resistivity decreases with the isoelectronic Cd substitution for Zn, contrary to the expectation for an increasingly disordered system, and implying that the valence shift is not related to charge carrier doping. The combined results demonstrate excellent complementarity between positive physical pressure and negative chemical pressure, and point to a rich playground for exploring the physics and chemistry of strongly correlated electron systems in the general family of Zn$_{20}$ compounds, despite their structural complexity.

cond-mat.str-el↗

arXiv2012

Ferromagnetic Instability in a Doped Band-Gap Semiconductor FeGa$_{3}$

We report the effects of electron doping on the ground state of a diamagnetic semiconductor FeGa$_{3}$ with a band gap of 0.5 eV. By means of electrical resistivity, magnetization and specific heat measurements we have found that gradual substitution of Ge for Ga in FeGa$_{3-y}$Ge$_{y}$ yields metallic conduction at a very small level of $y = 0.006$, then induces weak ferromagnetic (FM) order at $y = 0.13$ with a spontaneous moment of 0.1 $μ_{B}$/Fe and a Curie temperature $T_{C}= 3.3$ K, which continues increasing to $T_{C} = 75$ K as doping reaches $y = 0.41$. The emergence of the FM state is accompanied by quantum critical behavior as observed in the specific heat, $C/T \propto -$ln$T$, and in the magnetic susceptibility, $M/B \propto T^{-4/3}$. At $y= 0.09$, the specific heat divided by temperature $C/T$ reaches a large value of 70 mJ/K$^{2}$molFe, twice as large as that reported on FeSi$_{1-x}$Ge$_{x}$ for $x_{c}= 0.37$ and Fe$_{1-x}$Co$_{x}$Sb$_{2}$ for $x_{c}=0.3$ at their respective FM quantum critical points. The critical concentration $y_{c}=0.13$ in FeGa$_{3-y}$Ge$_{y}$ is quite small, despite the fact that its band gap is one order of magnitude larger than those in FeSi and FeSb$_{2}$. In contrast, no FM state emerges by substituting Co for Fe in Fe$_{1-x}$Co$_{x}$Ga$_{3}$ in the whole range $0 \leq x \leq 1$, although both types of substitution should dope electrons into FeGa$_{3}$. The FM instability found in FeGa$_{3-y}$Ge$_{y}$ indicates that strong electron correlations are induced by the disturbance of the Fe 3d - Ga 4p hybridization.

cond-mat.str-el↗

arXiv2008

Giant Uniaxial Anisotropy in the Magnetic and Transport Properties of CePd5Al2

Electrical resistivity rho, magnetic susceptibility chi, magnetization M and specific heat measurements are reported on a singlecrystalline sample of CePd5Al2, showing successive antiferromagnetic orderings at T_N1=4.1 K and T_N2=2.9 K. The temperature dependence of T_N1 shows a Kondo metal behavior with large anisotropy, rho_c/rho_a=3.2 at 20 K, and opening of a superzone gap along the tetragona c-direction below T_N1. Both T_N1 and T_N2 gradually increase with applying pressure up to 2.5 GPa. The data of chi(T) and M(B) in the paramagnetic state were analyzed using a crystalline electric field (CEF) model. It led to a Kramers doublet ground state with wave functions consisting primarily of |+-5/2>, whose energy level is isolated from the excited states by 230 and 300 K. This CEF effect gives rise to the large anisotropy in the paramagnetic state. In the ordered state, the uniaxial magnetic anisotropy is manifested as M_c/M_a=20 in B=5 T and at 1.9 K, and chi_c/chi_a=25 in B=0.1 T and at 4 K. This huge uniaxial magnetic anisotropy in the antiferromagnetic states can be interpreted in terms of isotropic magnetic interaction among the Ce^{3+} moments governed by the strong CEF. In powder neutron diffraction experiments, magnetic reflections were observed owing to the antiferromagnetic ordered states below T_N1, however, no additional reflection was found below T_N2.

cond-mat.str-el↗

arXiv2008

Neutron scattering study of Kondo lattice antiferromagnet YbNiSi3

The Kondo lattice antiferromagnet YbNiSi3 was investigated by neutron scattering. The magnetic structure of YbNiSi3 was determined by neutron diffraction on a single-crystalline sample. Inelastic scattering experiments were also performed on a pulverized sample to study the crystalline electric field (CEF) excitations. Two broad CEF excitations were observed, from which the CEF parameters were determined. The temperature dependence of the magnetic susceptibility chi and the magnetic specific heat Cmag were calculated using the determined CEF model, and compared with previous results.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic field tuning of the low temperature state in YbNiSi3

We present detailed, low temperature, magnetoresistance and specific heat data of single crystal YbNiSi3 measured in magnetic field applied along the easy magnetic axis, H || b. An initially antiferromagnetic ground state changes into a field-induced metamagnetic phase at ~16 kOe (T -> 0). On further increase of magnetic field, magnetic order is suppressed at ~85 kOe. The functional behaviors of the resistivity and specific heat are discussed in comparison with those of the few other stoichiometric, heavy fermion compounds with established field-induced quantum critical point.

cond-mat.str-el↗