arXiv subjects

R. Baquero

Publications and source records attributed to R. Baquero.

At least 19 recordsLinked to original sources

arXiv2019

High-T$_c$ superconductivity in H$_3$S: Pressure effects on superconducting critical temperature and Cooper-Pairs Distribution Function

We use first-principles calculations to study pressure effects on the vibrational and superconducting properties of H$_3$S in the cubic $Im\bar{3}m$ phase for the pressure range where the superconducting critical temperature (T$_c$) was measured (155-225 GPa). The pressure effects were incorporated using the Functional Derivative Method (FDM). In this paper, we present for the first time, the Cooper-Pairs Distribution Functions D$_{cp}$($ω,T_c$) for H$_3$S, which will allow to identify the spectral regions where Cooper-Pairs formation at temperature T$_c$ is more favorable. We analyzed in detail the pressure effects on the electron-phonon spectral density function $α^2$F$(ω)$ and phonon density of states (PhDOS) and their relationship with T$_c$. The FDM manages to reproduce the trend of the pressure dependence of critical temperature, in good agreement with experimental data in the range of 155 to 190 GPa. The $D_{cp}(ω,T_c)$ suggests that the low-frequency vibration region is where Cooper-Pairs are possible, which means that S-vibrations have an important role in the H$_3$S superconductivity properties.

cond-mat.supr-con↗

arXiv2018

The Thermodynamics and the Inverse Isotope Effect of superconducting PdH and PdD under pressure

We present in this paper the thermodynamics of superconducting PdH and PdD under pressure. We make use of a general method to calculate the thermodynamics under pressure within the Migdal-Eliashberg theory. We have considered the crystal lattice to be zincblende taking into account the experimental evidence for both PdH and PdD at temperatures below 55 K. We have studied, in particular, the changes induced by pressure in the critical temperature, $T_c$, in the specific heat jump at $T_c$, in the energy gap at $T=0K$, in the deviation function $D(t)$ and in the isotope effect coefficient, $α$. We get a very good agreement with experiment where this data exist. This method represents a basis on which the thermodynamics of other hydrides under pressure can be calculated.

cond-mat.supr-con↗

arXiv2018

On the Calculation of the Inverse Isotope Effect in PdH(D): A Migdal-Eliashberg Theory Approach

Replacement of hydrogen by deuterium in palladium hydride results in higher superconducting temperatures and an anomalous isotope effect that has not been yet fully explained. In this work, we try a new approach to the explanation of the inverse isotope effect in PdH(D). Our approach introduces two new aspects. First, we took into account the experimental evidence that at temperatures below 50 K, the crystal structure of PdH and of PdD is zincblende. Second, we take into account not only the influence of the electron-phonon interaction but also the electron-electron interaction contribution to the isotope coeffiecient due tothe replacement of deuterium in the place of hydrogen. We used the Migdal-Eliashberg theory to perform our ab initio calculations. We found in this picture that the electron-electron interaction is considerably reduced by the isotope substitution and is the most important factor to explain the inverse isotope effect. We found $ΔT_c^{total}=2.224\:K$ and $α= -0.3134$ in excellent agreement with the values found experimentally.

cond-mat.supr-con↗

arXiv2017

Inverse Isotope Effect in PdH(D)

In this letter, we take a new approach to the explanation of the inverse isotope effect in PdH(D). Our approach introduces two new aspects. First, we took into account the experimental evidence that at temperatures below 50 K, the crystal structure of PdH and of PdD is zincblende. Second, we studied the contribution of both, the electron-phonon and the electron-electron interactions. We used the Migdal-Eliashberg theory to perform our ab initio calculations. We found that the electron-electron contribution is the most important one to explain the inverse isotope effect. We reproduced the experimentally found values for the critical temperature and the isotope coefficient. Our analysis represents a direct and simple explanation for the inverse isotope effect in PdH(D).

cond-mat.supr-con↗

arXiv2017

Superconductivity under pressure: application of the functional derivative

In this paper, we calculate the superconducting critical temperature as a function of pressure, Tc(P ), using a method based on the functional derivative of the critical temperature with the Eliashberg function, dTc/dA2F. The coulomb electron-electron repulsion parameter, mu*(p) at each pressure is obtained in a consistent way by solving the linearized Migdal-Eliashberg equation. This method requires as the starting input only the knowledge of Tc(P ) at the starting pressure. It applies to superconductors for which the Migdal-Eliashberg equations hold. We study Al, a typical BCS weak coupling superconductor with a low Tc . Our results of Tc(P ) as a function of pressure for Al show an excellent agreement with the calculations of Profeta et al. (Phys. Rev. Lett.96, 047003 (2006)) which agree well with experiment.

cond-mat.supr-con↗

arXiv2015

Effects of Pb doping on structural and electronics properties of Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10}$

Pb doping effect in the Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10}$ compound (Bi2223) on the structural and electronic properties were investigated, using the Local Density (LDA) and Virtual Crystal (VCA) approximations within the framework of the Density Functional Theory (DFT), taking as reference the procedure implemented by H.Lin {\it et al.} in the Bi2212 compound [{\it Phys. Rev. Lett.} {\bf 96} (2006) 097001]. Results show that, the incorporation of Pb-dopant in Bi2223 lead a rigid displacement of the Bi/Pb-O bands toward higher energies, with a null contribution at the Fermi level, around the high symmetry point $\overline{\text{M}}$ in the irreducible Brillouin zone, for Pb doping concentration equal to or more than 26\%, avoiding the presence of the so-called Bi-O {\it pockets} in the Fermi surface, in good agreement with angle-resolved photoemission spectroscopy (ARPES) and nuclear magnetic resonance (NMR) experiments, although a slight metallic character of the Bi-O bonds is still observed which would disagree with some experimental reports. The calculations show that the changes on the structural properties are associated to the presence or absence of the Bi-O {\it pockets} in the Fermi surface

cond-mat.str-el↗

arXiv2013

Performance of the modified Becke-Johnson potential

Very recently, in the 2011 version of the Wien2K code, the long standing shortcome of the codes based on Density Functional Theory, namely, its impossibility to account for the experimental band gap value of semiconductors, was overcome. The novelty is the introduction of a new exchange and correlation potential, the modified Becke-Johnson potential (mBJLDA). In this paper, we report our detailed analysis of this recent work. We calculated using this code, the band structure of forty one semiconductors and found an important improvement in the overall agreement with experiment as Tran and Blaha [{\em Phys. Rev. Lett.} 102, 226401 (2009)] did before for a more reduced set of semiconductors. We find, nevertheless, within this enhanced set, that the deviation from the experimental gap value can reach even much more than 20%, in some cases. Furthermore, since there is no exchange and correlation energy term from which the mBJLDA potential can be deduced, a direct optimization procedure to get the lattice parameter in a consistent way is not possible as in the usual theory. These authors suggest that a LDA or a GGA optimization procedure is used previous to a band structure calculation and the resulting lattice parameter introduced into the 2011 code. This choice is important since small percentage differences in the lattice parameter can give rise to quite higher percentage deviations from experiment in the predicted band gap value.

cond-mat.mtrl-sci↗

arXiv2013

Detailed analysis of the Bi-O pockets problem in $Bi_2Sr_2Ca_2Cu_3O_{10}$

The Bi-O pockets problem, namely, the appearance in theoretical ab initio calculations of the electronic band structure of Bi-cuprates of a pocket of states at the Fermi energy ($E_F$) that is attributed to states belonging to the Bi-O plane is an issue that still calls for more study. The Bi-O pockets are in contradiction with experiments. We have investigated the possible reasons for the disagreement. We checked that by using the experimental lattice and internal parameters without any optimization procedure, the Bi-O pockets do not appear at $E_F$ in agreement with experiment. Nevertheless, as pointed out by R. Kouba et al. [{\em Phys. Rev. B} {\bf 60}, 9321 (1999)] optimization is compulsory to a band structure calculation that will describe appropriately the electronic properties. But starting with the experimental parameters a further optimization procedure previous to the actual ab initio calculation leads to the Bi-O pockets. Doping with 25% of Pb they disappear. From the several configurations that we have considered, we found two very simple ways in which the Bi-O pockets disappear without avoiding an optimization procedure previous to the calculation and without including a doping of any kind. In this paper, we report the effect of the slight displacement of the oxygen atom associated to the Sr-plane (O3) in the electronic properties of $Bi_2Sr_2Ca_2Cu_3O_{10}$ (Bi-2223) with tetragonal structure ($I4/mmm$) using the Local Density Approximation (LDA). The slight displacement is performed after the system has been optimized. We determined the intervals of the O3 atomic positions for which calculations of the band structures show that the Bi-O bands emerge towards higher energies in agreement with the experimental results, thereby solving the Bi-O pockets problem (continue).

cond-mat.str-el↗

arXiv2013

First-principles study of electronic structure of $Bi_2Sr_2Ca_2Cu_3O_{10}$

We present for the first time the band structure calculation of $Bi_2Sr_2Ca_2Cu_3O_{10}$ compound in the tetragonal structure (space group $I4/mmm$). We used the Local Density Approximation (LDA) as in the Wien2k code. We analyze in detail the band structure and the Fermi surface (FS). Our results are in very good agreement with recent experiments. The FS shows the feature known as the Bi-O pocket problem which we associate with the interaction of the O3 atoms with the Cu2-O2 and Bi-O4 planes. Ceramic $Bi_2Sr_2Ca_2Cu_3O_{10}$ stabilized with Pb has been reported as a superconductor with $T_c \sim 100$. CdS microparticles were embedded into the ceramic $Bi_2Sr_2Ca_2Cu_3O_{10}$. The composite did show a superconducting phase transition at a lower $T_c \sim 70 K$. At even lower temperatures re-entrant behavior was observed. The sample regain the superconducting state at $\sim 47 K$ [arXiv:1101.0277 [cond-mat.supr-con]]. This effect is not observe in the ceramic alone. This calculation is useful per ser and also can contribute to a better understanding of this particular re-entrant behavior.

cond-mat.str-el↗

arXiv2013

The modified Becke-Johnson potential analyzed

Recently in the Wien2k code, the modified Becke-Johnson potential (mBJLDA) was implemented. As the authors [{\em Phys.Rev.Lett.} 102, 226401 (2009)] point, this potential reproduces the band gap of semiconductors with improved accuracy. In this paper we present our analysis of this potential in two directions. First, we checked whether this potential reproduces the band structure for metals, an analysis that lacked in the literature. We calculated the band gap of a group of semiconductors. We observed that the Linear Density Approximation (LDA) give rise to a shorter lattice constant as compared to experiment. The Generalized Gradient Approximation behaves oppositely. Using the average, $a_{Avg}$, in the mBJLDA potential, we obtained a closer to experiment value for the gap. We conclude that the new mBJLDA potential represent an important improvement as compared to the results from the previous version of the Wien2k code. Also the mBJLDA potential can be a very useful tool for the theoretical study of complex systems containing semiconductor compounds such as surfaces, superlattices and interfaces.

cond-mat.mtrl-sci↗

arXiv2013

The band gap problem: the accuracy of the Wien2k code confronted

This paper is a continuation of our detailed study [Phys. Rev. B 86, 195106 (2012)] of the performance of the recently proposed modified Becke-Jonhson potential (mBJLDA) within the known Wien2k code. From the 41 semiconductors that we have considered in our previous paper to compute the band gap value, we selected 27 for which we found low temperature experimental data in order to pinpoint the relative situation of the newly proposed Wien2k(mBJLDA) method as compared to other methods in the literature. We found that the GWA gives the most accurate predictions. The Wien2k (mBJLDA) code is slightly less precise, in general. The Hybrid functionals are less accurate, on the overall. The GWA is definitely the most precise existing method nowadays. In 88% of the semiconductors considered the error was less than 10%. Both, the GWA and the mBJLDA potential, reproduce the band gap of 15 of the 27 semiconductors considered with a 5% error or less. An extra factor to be taken into account is the computational cost. If one would seek for precision without taking this factor into account, the GWA is the method to use. If one would prefer to sacrifice a little the precision obtained against the savings in computational cost, the empirical mBJLDA potential seems to be the appropriate method. We include a graph that compares directly the performance of the best three methods, according to our analysis, for each of the 27 semiconductors studied. The situation is encouraging but the problem is not yet a closed issue.

cond-mat.str-el↗

arXiv2012

Metallic behavior at YBaCuO7/ZAs interfaces (Z=Ga, Al)

We present the electronic band structure of the interfaces $YBa_2Cu_3O_7/GaAs$ (direct gap) and $YBa_2Cu_3O_7/AlAs$ (indirect gap) in different configurations calculated using the Density Functional Theory as in the Wien2k code within the local density approximation. We have projected the density of states at the atomic layers forming the interface. We concentrated in the semiconductor side. The two first atomic layers in the semiconductor side of the interface present a clear metallic behavior. We found for both semiconductors considered that it converges towards the bulk atomic-layer projected density of states at the fifth atomic layer from the interface. We considered an ideal non-reconstructed interface in the (001) direction in this work. This behavior is interesting and could be used in several technological applications.

cond-mat.mtrl-sci↗

arXiv2010

Processing and study of the composite CdS/Bi-Pb-Sr-Ca-Cu-O

We have fabricated and characterized samples of the superconducting- semiconducting Bi-Pb-Sb-Sr-Ca-Cu-O/CdS composite. Nano-size particles of CdS were deposited and introduced into the porosities of the Bi-Pb-Sb-Sr-Ca-Cu-O material by the spray pyrolysis technique. The morphology and hollow size in the porous superconducting material as well as the grain size in CdS and the morphology of the surface of the composite were obtained by Scanning Electron Microscopy. We obtained the critical superconducting temperature of both the Bi-Pb-Sb-Sr-Ca-Cu-O and the Bi-Pb-Sb-Sr-Ca-Cu-O/CdS composite measuring the resistivity. Both show a metallic behaviour just above the superconducting transition. For the superconductor alone, resistivity starts falling at Tc,on sup = 99,9 K and reaches zero at Tc,sup=76,3 K. The behaviour of the composite is different. The transition starts at Tc,on comp = 65,3 K and reaches zero resistance at Tc,comp = 56,5 K. This seems to indicate that the semiconductor penetrates the whole superconducting Bi-Pb-Sb-Sr-Ca-Cu-O sample so that there is no region of pure superconducting material left. Since the materials do not actually mix (see text) the behaviour might be attributable to the interface. Also the resistivity curves present a very interesting feature, i.e., below the temperature at which the composite attains zero-resistivity, a re-entrant behaviour manifests itself and a finite resistivity peak appears. It increases to a certain value to drop back to zero at some temperature below. We comment further on this feature in the text.

cond-mat.supr-con↗

arXiv2005

(100) ideal-surface band structure for the series of Cu-based chalcopyrites

We use the Surface Green Function Matching (SGFM) method and a tight-binding hamiltonian to calculate the (100)-surface electronic band structure and local density of states of the series of Cu-based A^{I}B^{III}C2^{VI} chalcopyrites . We find four surface states in the optical gap energy region of s-p character and three surface states in the conduction band region of p-character. We show the trends of different characteristics within the series by means of figures and tables so that the quantitave behavior can be evaluated as well. We did not find Frontier Induced Semi-Infinite states of non-dispersive character in the studied range of energy within the valence band as we found in the case of the (112) surface electronic band structure for CuInSe2.

cond-mat.mtrl-sci↗

arXiv2005

The Optical conductivity resonance from an exact description of the electronic states around the Fermi energy

In this paper we show that the optical conductivity can be calculated to agree with experiment if the details of the electronic states around the Fermi level are taken into account with some care. More precisely, we present a calculation of the optical conductivity in YBa2Cu3O7 on the basis of an exact (ab initio) three dimensional electronic band structure calculation from which we extract the information on the bands near the Fermi energy that can be associated to the CuO2 plane-carrier states. To simulate the superconducting state we superimpose a gap to these bands alone. On these basis, we calculate from the known Kubo-Greenwood formula, the optical conductivity in the normal and in the superconducting state. Our calculation agrees with the experimental result even in the higher part of the frequency spectrum. Our way of calculating the resonance suggests a model of evolution for the bands under the effect of doping consistent with the recent experimental findings that the optical resonance can disappear while the sample remains superconducting. An important conclusion of this paper is that the resonance depends mostly on the details of the electronic band structure. It is enough to take into account the effect of the superconducting transition through a single parameter (the gap). No details on the mechanism are needed so no mechanism can be tested on this basis. Our calculation suggests a model of evolution for the bands around the Fermi energy under doping that gives some microscopic foundations to the the recent experiments that show unambiguously that the resonance cannot be the cause of superconductivity.

cond-mat.supr-con↗

arXiv2005

Influence of distortion on the electronic band structure of CuInSe2

We present a tight-binding calculation of the influence of distorsion on the bulk electronic structure of the chalcopyrite CuInSe2. We calculate the ideal case and then the effect of the inclusion of the distortions. We analyze our results in detail and conclude from a comparison with other work that the distortions must be included in the Hamiltonian to get a proper account of the electronic band structure. We use our new Hamiltonian to study the effect that both the tetragonal and the anionic distortion have on the (112) surface electronic band structure. We find this effect non-negligible.

cond-mat.mtrl-sci↗

arXiv2000

Ferromagnetism in a (001), (110), and (111)-oriented Ru mololayer on Ag, Au, and Cu-substrates

We have studied the magnetic behavior of a 4d transition metal Ru monolayer (ML) on different substrates and orientations. In the ground state, a Ru-ML is expected to be ferromagnetic on Ag(001) and Au(001) with a magnetic moment $(μ) $ of 1.73 Bohr magnetons $(μ_{B}) $ in both cases. On Cu(001), a Ru-ML is not magnetic. In this paper, we study the magnetic behavior of a Ru-ML at other orientations, i.e., (110) and (111). We found magnetism on Au (111), and Ag (111) ($μ\thicksim 1.3μ_{B}$ for both) but no magnetic activity on a Cu substrate in any orientation. Further, we found that on Ag(110), a Ru-ML is ferromagnetic with $μ=1.3μ_{B}$. On Au(110), a Ru-ML is not magnetic. Since on the (001) and (111) orientations a Ru-ML has about the same magnetic activity on Ag as on Au, we found surprising the behavior in the (110) orientation. We analyze it in detail in the final part of the paper. We found that there exists a metastable ferromagnetic state, in this case, and that a Ru-ML becomes ferromagnetic under a small expansion of the Au-lattice. This is not the case for Cu.

cond-mat.mtrl-sci↗

arXiv1999

Van Hove Singularities around the Fermi level in YBa2Cu3O7: The importance of the chains

We have reproduced band structure calculations from the literature and have used them to analyze in detail the energy landscape around the Fermi level. We found three Van Hove singularities, two below (-230, -54 meV) and one above the Fermi level (+27 meV). We have studied the composition of each one of them and found that states comming from the chain do contribute in a very importan way. The contribution from the planes are indeed important and, therefore, we find that a 2D description includes the most important contributions. Nevertheless, the contribution from states out of the planes (the chains and the apical oxygen) is by no means negligable. We find that it is possible that in YBaCuO part of the condensate lies in the chains, a fact that would agree with some recent evidences concerning PrBaCuO. Our general conclusion is that the 2D description of 123-compounds might be insuficient to explain all the experimental details and that a 3D description seems compulsory to fully account for the phenomenom of superconductivity in YBaCuO.

cond-mat.mtrl-sci↗