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M. S. da Luz

Publications and source records attributed to M. S. da Luz.

14 recordsLinked to original sources
arXiv2026

Multiband Superconductivity and Charge Density Wave in HfxZr1-xTe3 single crystals

We report on the bulk multiband superconductivity and charge density wave (CDW) in HfxZr1-xTe3 single crystals. The parent compound ZrTe3 is a layered van der Waals material that undergoes a CDW transition near 63 K and exhibits only filamentary superconductivity below 2.0 K. Upon the introduction of a small amount of Hf (x = 0.02), the CDW transition temperature is reduced to TCDW ~ 53 K, while a robust bulk superconducting state emerges at Tc ~ 3.3 K, underscoring a subtle competition between CDW order and superconductivity in this quasi-one-dimensional system. Electrical resistivity, magnetic susceptibility, Hall effect, Seebeck coefficient, and specific heat measurements consistently confirm the bulk nature of the superconducting phase. The temperature dependence of the upper critical field Hc2(T) deviates markedly from single-band behavior, and it is well described by a two-band model, consistent with multiband superconductivity. Analysis of the Hall effect, and thermoelectric behavior reveal pronounced electronic anisotropy, with enhanced effective carrier masses, indicating that the subtle structural modification introduced by Hf substitutions affects the Fermi surface topology, as well as electronic correlations. Measurements of electrical resistivity in hydrostatic pressures up to ~ 2 GPa reveal that pressure drives TCDW to higher temperatures while suppressing Tc. These findings show that Hf doping can be used to fine-tune the balance between the CDW instability and superconductivity, possibly by means of chemical pressure effects, stabilizing a multiband superconducting state in Hf-doped ZrTe3.

cond-mat.supr-con↗
arXiv2026

Substitution modulated transition from semimetal to superconductor in ZrTe$_{2-x}$Se$_x$ with coexistence of nontrivial electronic topology

This study explores the emergence of superconductivity in high-quality ZrTe$_{2-x}$Se$_x$ crystals, grown via the isothermal chemical vapor transport (ICVT) technique. Resistive, structural, and thermal measurements reveal that substituting Te with Se in the ZrTe$_2$ matrix induces a superconducting state at low temperatures. The critical temperature ($T_c$) exhibits a clear dependence on the selenium concentration, peaking at $x=0.15$ with a $T_c$ of $4.8$ K. Calorimetric data indicates that even a low Se substitution range is capable of modifying both the electronic contribution and the vibrational modes of the crystal lattice. Combined with ab initio calculations and Wannier Hamiltonian interpolation between ZrTe$_2$/ZrSe$_2$, we established an extensive phase diagram mapping the transition from charge density wave (CDW) to the state with coexistence between the Dirac semimetal and superconductivity (SC), up to the semiconductor phase. This coexistence suggests that ZrTe$_{1.85}$Se$_{0.15}$ could be a candidate platform for topological superconductivity, as it hosts a nontrivial $\mathbb Z_2$ invariant, with nonvanishing surface states in its $(001)$ planes.

cond-mat.supr-con↗
arXiv2023

Superconductivity in Te-deficient ZrTe$_2$

We present structural, electrical, and thermoelectric potential measurements on high-quality single crystals of ZrTe$_{1.8}$ grown from isothermal chemical vapor transport. These measurements show that the Te-deficient ZrTe$_{1.8}$, which forms the same structure as the non-superconducting ZrTe$_2$, is superconducting below 3.2\,K. The temperature dependence of the upper critical field (H$_{c2}$) deviates from the behavior expected in conventional single-band superconductors, being best described by an electron-phonon two-gap superconducting model with strong intraband coupling. For the ZrTe$_{1.8}$ single crystals, the Seebeck potential measurements suggest that the charge carriers are predominantly negative, in agreement with the ab initio calculations. Through first-principles calculations within DFT, we show that the slight reduction of Te occupancy in ZrTe$_2$ unexpectedly gives origin to density of states peaks at the Fermi level due to the formation of localized Zr-$d$ bands, possibly promoting electronic instabilities at the Fermi level and an increase at the critical temperature according to the standard BCS theory. These findings highlight that the Te deficiency promotes the electronic conditions for the stability of the superconducting ground state, suggesting that defects can fine-tune the electronic structure to support superconductivity.

cond-mat.supr-con↗
arXiv2022

A Simple and Precise Way to Determine Electrical Resistivity of Isotropic Conductors: Simplifying the Four-Probe Method

COMSOL Multiphysics software is used to describe the behavior of the electrical resistivity of several samples with rectangular shape typically used in the Montgomery method. The simulation data obtained using four isotropic conductors allowed us to understand in detail the behavior of the electric potential and electric field of the samples. The results provide an analytical method which can substitute the four-probe method with much more simplicity and precision.

cond-mat.mtrl-sci↗
arXiv2022

On the Fermi gas, the Sommerfeld fine structure constant, and the electron-electron scattering in conductors

Electrical energy is considered as a fundamental parameter for inclusion in Fermi gas theory, in addition to thermal energy. It is argued that electrical energy can move some electrons to above the Fermi Level, providing free charges to carry the electrical current, even at absolute zero temperature. The Drude model, Ohm's law, quantum resistance, and the electrical resistivity due to electron-electron scattering appear naturally as a consequence of the theoretical description, which is based on the quantization of the angular momentum and the Fermi-Dirac distribution, considering total energy as $ε$ = k$_B$$T$ + $Φ_0$$I$. The electrical and magnetic forces acting on an electron are related to the ratio between the Fermi velocity and the speed of light and show that the electron motion is due to helical paths. Considering the center of mass description for the Bohr atom, it was possible to show that the magnetic force is related to the electrical force as $F_M$ = ($α$/$π$) $F_E$, which demonstrates that the electrons move in helical paths along the orbit. The helical motion naturally provides for quantization of the magnetic flux, the spin of the electron, and the first correction term of the anomalous magnetic moment. Applying the model to describe the electron-electron scattering allows prediction of the behavior of the electrical resistivity of many metals at low temperatures, which is in excellent agreement with empirical observations.

cond-mat.mtrl-sci↗
arXiv2021

On the Order Parameter of the Continuous Phase Transition in the Classical and Quantum Mechanical limits

The mean field theory is revisited in the classical and quantum mechanical limits. Taking into account the boundary conditions at the phase transition and the third law of the thermodynamics the physical properties of the ordered and disordered phases were reported. The equation for the order parameter predicts the occurrence of a saturation of $Ψ^2$ = 1 near $Θ_S$, the temperature below the quantum mechanical ground state is reached. The theoretical predictions are also compared with high resolution thermal expansion data of SrTiO$_{\text{3}}$ monocrystalline samples and other some previous results. An excellent agreement has been found suggesting a universal behavior of the theoretical model to describe continuous structural phase transitions.

cond-mat.other↗
arXiv2018

Evidence of unconventional superconductivity in the Ni-doped NbB2 system

In this paper, a comprehensive study of the effects of Ni-doping on structural, electrical, thermal and magnetic properties of the NbB2 is presented. Low amounts (\leq 10 %) of Ni substitution on Nb sites cause structural distortions and induce drastic changes in the physical properties, such as the emergence of a bulk superconducting state with anomalous behaviors in the critical fields (lower and upper) and in the specific heat. Ni-doping at the 9 at.% level, for instance, is able to increase the critical temperature (TC) in stoichiometric NbB2 (< 1.3 K) to approximately 6.0 K. Bulk superconductivity is confirmed by magnetization, electronic transport, and specific heat measurements. Both Hc1 and Hc2 critical fields exhibit a linear dependence with reduced temperature (T/TC), and the specific heat deviates remarkably from the conventional exponential temperature dependence of the single-band BCS theory. These findings suggest multiband superconductivity in the composition range from 0.01 \leq x \leq 0.10.

cond-mat.supr-con↗
arXiv2014

Interplay between antiferrodistortive, ferroelectric and superconducting instabilities in Sr_{1-x}Ca_{x}$TiO_{3-δ}

SrTiO$_{3}$ undergoes a cubic-to-tetragonal phase transition at 105K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical non-isovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is an open question, which gathers momentum in the context of the growing popularity of the paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering and dielectric permittivity and polarization measurements on Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ ($0<x<0.009$) and a low-temperature electric conductivity in Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-δ}$. Calcium substitution was found to enhance the transition temperature for both anti-ferrodistortive and ferroelectric transitions. Moreover, we find that Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-δ}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent, is slightly lower than in SrTiO$_{3-δ}$ of comparable carrier concentration. A three-dimensional phase diagram for Sr$_{1-x}$Ca$_{x}$TiO$_{3-δ}$ tracking the three transition temperatures as a function of x and $δ$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbours.

cond-mat.str-el↗
arXiv2006

Orthorhombic to tetragonal phase transition and superconductivity in the Ba2Cu3O4Cl2 compound

In this work we have investigated the orthorhombic to tetragonal phase transition in the Ba2Cu3O4Cl2 compound. This transition was observed by X-ray powder diffractometry carried out in samples heat treated between 700 and 750OC and also in samples with Ba2ZnCu2O4Cl2 composition. Results of X-ray diffractograms simulation confirm the phase transition. dc-Magnetization measurements performed in SQUID showed the existence of diamagnetism signal. The results suggest the existence of localized superconductivity and can explain the different magnetic properties reported in literature for the Ba2Cu3O4Cl2 compound.

cond-mat.mtrl-sci↗
arXiv2006

Superconductivity in the Nb2SnC compound

Nb2SnC is a member of the large family of lamellar materials that crystallize in the hexagonal structure with space group P63/mmc which are isomorphs with Cr2AlC, also named H-phase. In spite of the great number of compounds which belong to this family, the superconductivity has been reported only for two cases: Mo2GaC and Nb2SC. In this work we show that superconductivity can be observed in Nb2SnC depending on the synthesis method used. The quality of the superconductor is strongly dependent of the synthesis method and the optimal results were reached for samples synthesized at 2.5 GPa and 523 +/- 50oC. This sample showed a critical temperature close to 7.8K, revealed from magnetization and transport measurement, the highest critical temperature reported up to now for an H-phase.

cond-mat.supr-con↗
arXiv2006

Transport Properties of Granular High-TC Superconductors

We report on the application of the Resistively Shunted Junction (RSJ) model to granular high-TC superconductors. Some derived predictions of the RSJ model are applied to a set of superconducting granular samples which can be considered as a network of Josephson junctions. The investigated samples belong to both hole-doped Y1-xPrxBa2Cu3O7-d (x = 0.0, 0.35, and 0.45) and the electron-doped Sm2-xCexCuO4-d (x = 0.18) systems which display the so-called double resistive superconducting transition. We have performed several transport measurements in these compounds including temperature and magnetic field dependence of the electrical resistance, R(T,H), and I-V characteristics. Several aspects of the I-V characteristics were quantitatively well described by the RSJ model. The combined results strongly suggest that dissipation in granular superconducting samples is a natural consequence of the normal current flowing in parallel with the supercurrent current.

cond-mat.supr-con↗
arXiv2006

Observation of double superconducting transition in Y0.55Pr0.45Ba2Cu3O7-d polycrystalline sample by Hall effect measurement

This work reports longitudinal (RXX) and transverse (RXY) resistance as a function of the temperature measured at low magnetic fields for a Y0.55Pr0.45Ba2Cu3O7-d polycrystalline sample. It is observed nonzero transverse voltage at zero external magnetic below superconducting transition and above no such voltage was detected. Comparing RXX (T) and RXY (T) curves it was possible to observe a correlation between Hall resistance and double resistive superconducting transition. Magneto-resistance RXX(H) measurements showed that the dissipation is dominated by weak coupling between superconducting clusters. I-V and R(T) curves of both components suggest that the positive Hall voltage at superconducting state must be related to quasiparticle currents.

cond-mat.supr-con↗
arXiv2006

Two-Fluid Model for Granular Superconductors

A two-fluid model is proposed to describe the transport properties of granular superconductors. Using the resistively shunted junction model and some aspects of the two-level system theory, a statistical model is developed which takes into account the ratio between the number of normal and superconducting electrons carrying the applied current. The theoretical model reveals excellent agreement when compared to transport properties of four high-Tc superconductors. The results suggest that the two-fluid model is independent of the sample composition, critical temperature and whether the superconducting compound is electron or hole-doped.

cond-mat.supr-con↗