arXiv subjects

G. Bouzerar

Publications and source records attributed to G. Bouzerar.

At least 19 recordsLinked to original sources

arXiv2025

Crossing over from flat band superconductivity to conventional superconductivity

Over the past ten years, flat band (FB) or geometric superconductivity has become a major issue in condensed matter physics due to the significant technological benefits it could offer. Observations of this unconventional form of superconductivity are unfortunately still very limited, and significant efforts are being made to search for candidate materials. Most existing theoretical studies focus on systems with strictly non-dispersive bands, which, from an experimental point of view, represents an extremely difficult technological constraint to achieve. It is therefore crucial to understand to what extent this constraint can be relaxed. In other words, to what extent can superconductivity in flat bands survive weak perturbations? The main objective of the present study is precisely to answer this essential question in detail.

cond-mat.supr-con↗

arXiv2024

Strengthening of the superconductivity by real space decimation of the flat band states

In contrast to standard BCS superconductivity, that in flat bands (FBs) possesses an interesting degree of freedom that enables the control of the superfluid weight (SFW), referred to as the quantum metric (QM). In the present work, we consider the stub lattice and study the impact of the dilution of FB eigenstates on superconductivity. Among the most remarkable results, it is revealed that the SFW can be boosted by the decimation of the FB eigenstates. In addition, it is shown that the widely used uniform pairing hypothesis systematically predicts the suppression of the SFW, appears misleading and qualitatively incorrect. With the great progress in nanotechnologies, we believe that our findings could be realised and tested experimentally in covalent organic frameworks or in decorated structures in which defects/vacancies/ad-atoms are created/deposited in a controlled manner and even in multilayered structures with intercalated atoms.

cond-mat.supr-con↗

arXiv2023

Hidden symmetry of Bogoliubov de Gennes quasi-particle eigenstates and universal relations in flat band superconducting bipartite lattices

Unconventional flat band (FB) superconductivity, as observed in van der Waals heterostructures, could open promising avenues towards high-T$_c$ materials. In FBs, pairings and superfluid weight scale linearly with the interaction parameter, such an unusual behaviour justifies and encourages strategies to promote FB engineering. Bipartite lattices (BLs) which naturally host FBs could be particularly interesting candidates. Within Bogoliubov de Gennes theory and in the framework of the attractive Hubbard model in BLs, a hidden symmetry of the quasi-particle eigenstates is revealed. As a consequence, we demonstrate universal relations for the pairings and the superfluid weight that are independent of the characteristics of the hopping term. Remarkably, it is shown that these general properties are insensitive to disorder as long as the bipartite character is protected.

cond-mat.supr-con↗

arXiv2023

Constrain relations for superfluid weight and pairings in a chiral flat band superconductor

Within ten years, flat band (FB) superconductivity has gained a huge interest for its remarkable features and connection to quantum geometry. We investigate the superconductivity in a FB system whose orbitals are inequivalent and in which the gap and the quantum metric are tunable. The key feature of the present theoretical study is to show a unique and simple constrain relation that pairings obey. Furthermore, pairings and superfluid weight in partially filled FB are shown to be controlled by those of the half-filled lattice. We argue that the geometry of the lattice or the complexity of the hopping terms have no impact on the features revealed in this work as far as the system is bipartite.

cond-mat.supr-con↗

arXiv2022

Flat band induced room temperature ferromagnetism in two dimensional systems

The remarkable and fascinating properties of two-dimensional materials have raised them to the rank of most promising candidates for technological applications. In particular, the possibility of long-range ferromagnetic order in 2D materials is generating a growing excitement. Here, we demonstrate that flat bands (FBs) may pave the way to room temperature ferromagnetism in 2D compounds. Indeed, the magnetic exchanges between localized spins are largely dominated by the FB-FB contribution. This contribution is ferromagnetic and scales linearly with the local coupling, thus leading to cross-over temperatures (ferromagnetic phase/ paramagnetic phase) higher by an order of magnitude than those currently reported in experiments. Cross-over temperatures as high as $600~ K$ could be reached in micrometer-sized FB systems.

cond-mat.str-el↗

arXiv2022

Giant boost of the quantum metric in disordered one dimensional flat band systems

It is a well known fact, that the disorder has its most dramatic effects on the conventional quantum transport in one dimensional systems. In flat band (FB) systems, it is revealed that the conductivity at the FB energy is robust against the disorder and can even be tremendously boosted. Furthermore, challenging our understanding of the physical phenomena, the giant increase occurs in the limit of low FB states density. The singular behaviour of the quantum metric of the FB eigenstates is found to be at the heart of these unexpected and puzzling features. Our findings should have interesting fallout for other physical systems, and may as well open up engineering strategies to boost the critical temperature in two dimensional superconducting FB materials.

quant-ph↗

arXiv2020

d0 Ferromagnetism in Mg-doped Rutile TiO2 Nanoparticles

In a quest of enriching the area of d0 magnetism in oxide materials, we have undertaken to study Mg-doped TiO2 compounds. The Ti1-xMgxO2 (x=0, 0.02, 0.04 and 0.06) nanoparticles were prepared by solid-state reaction route. The X-ray diffractions (XRD) patterns of these samples indicate single phase of tetragonal rutile-structure of TiO2. The refinement of the XRD patterns reveals no change in the crystallographic lattice parameters in comparison to pure TiO2 upon Mg doping and it indicates that Mg2+ ions do not enter core grains and form core/shell structure. SEM observations reveal the uniform morphology with nanometric grains in the range of 150-200 nm. The measurement of magnetic properties of these compounds indicates that pure TiO2 and Ti0.98Mg0.02 compounds exhibit paramagnetic behavior and Ti0.96Mg0.04 compound exhibits ferromagnetic (FM) phase superimposed with the dominating paramagnetic phase. However, Ti0.94Mg0.06 compound exhibits ferromagnetic to paramagnetic transition with FM transition temperature of 180.2 K. The measurements of zero field and field cooled magnetization data indicate low temperature magnetic irreversibility for x=0.06 sample and it was attributed to the competing AFM (core) and the FM (shell) interactions. The measurement of hysteresis curves at various temperatures indicates domain wall pinning and an exchange-bias behavior.

cond-mat.mtrl-sci↗

arXiv2020

d0 Ferromagnetism in Li-doped ZnO Compounds

Recently, d0 ferromagnetic materials have been projected as one of the promising novel materials for spintronics applications. In this work, we have studied Li-doped ZnO compounds, i.e. Zn1-xLixO (x=0, 0.02, 0.04, and 0.06) samples, prepared by the solid-state reaction route method. From the study of crystal structure using X-ray diffraction (XRD) patterns, it is evident that the prepared materials have been formed in a single-phase of the hexagonal wurtzite structure. The refinement of the XRD patterns suggests that there are very small changes in the lattice parameters upon Li-incorporation in ZnO. The average crystallite size (SC), estimated from XRD patterns was found to be in the range of 35-50 nm. The microstructural study by scanning electron microscope reveals the uniform morphology of the grains of the order of 50-70 nm. The energy dispersive spectrum indicates that no unwanted ferromagnetic impurities have crept into the final prepared samples. The measurement of the temperature (T) variation of magnetization (M) with SQUID magnetometer indicates that undoped ZnO exhibits diamagnetic property but all Li-doped compounds exhibit room-temperature ferromagnetism and with a magnetic irreversibility behavior between zero-field cooled and field cooled M-T data. From the magnetization versus field measurements at 3 and 300 K, it is observed that Li-doped samples exhibit ferromagnetic loops with ultra-soft coercivity (~50 Oe) and with a maximum saturation magnetization of 0.10 emu/gm for x= 0.02 sample, which decreases with the increase in Li concentration.

cond-mat.mtrl-sci↗

arXiv2020

d0 Ferromagnetism in Ag-doped Monoclinic ZrO2 Compounds

Recently d0 or intrinsic ferromagnetism was believed to provide an alternative pathway to transition metal induced ferromagnetism in oxide. In pursuit of augmenting the area of d0 ferromagnetism; we have undertaken to study the crystal structure and magnetic properties of Ag-doped ZrO2 compounds. Polycrystalline samples of Zr1-xAgxO2 (with x=0, 0.02, 0.04, 0.06 and 0.08) were prepared by solid-state reaction route. All the prepared compounds are found to crystallize in monoclinic symmetry of ZrO2. In our study, pure ZrO2 compound exhibits paramagnetic behavior. However, the Ag-doped ZrO2 compounds exhibit ferromagnetic to paramagnetic transition. The Curie temperature was found to increase from 28.7 K for x=0.02 to 173.2 K for x= 0.08 doped ZrO2. Thus, the introduction of Ag in ZrO2 induces ferromagnetism with a large ThetaC. The measurements of hysteresis curves indicate that Ag doped ZrO2 compounds exhibit hysteresis loops with a coercivity of around 1350 Oe. Moreover, increase in Ag concentration resulted increase in the value of saturation magnetization (MS); the maximum value of MS was recorded as 0.01 μB/Ag ion for x= 0.06 sample. The sintering of sample at high temperature (13500C) diminishes the ferromagnetism and it leads to paramagnetic behaviour.

cond-mat.mtrl-sci↗

arXiv2020

Quantum transport in flat bands and super-metallicity

Quantum physics in flat-band (FB) systems embodies a variety of exotic phenomenon and even counter intuitive features. The quantum transport in several graphene based compounds that exhibit a flat band and a tunable gap is investigated. Despite the localized nature of the FB states and a zero group velocity, a super-metallic (SM) phase at the FB energy is revealed. The SM phase is robust against the inelastic scattering strength and controlled only by the inter-band transitions between the FB and the dispersive bands. The SM phase appears insensitive and quasi independent of the gap amplitude and nature of the lattice (disordered or nano-patterned). The universal nature of the unconventional FB transport is illustrated with the case of electrons in the Lieb lattice.

cond-mat.mes-hall↗

arXiv2019

Dramatic effects of vacancies on phonon lifetime and thermal conductivity in graphene

Understanding thermal transport in 2D materials and especially in graphene is a key challenge for the design of heat management and energy conversion devices. The high sensitivity of measured transport properties to structural defects, ripples and vacancies is of crucial importance in these materials. Using a first principle based approach combined with an exact treatment of the disorder, we address the impact of vacancies on phonon lifetimes and thermal transport in graphene. We find that perturbation theory fails completely and overestimates phonon lifetimes by almost two orders of magnitude. Whilst, in defected graphene, LA and TA modes remain well defined, the ZA modes become marginal. In the long wavelength limit, the ZA dispersion changes from quadratic to linear and the scattering rate is found proportional to the phonon energy, in contrast to the quadratic scaling often assumed. The impact on thermal transport, calculated beyond the relaxation time approximation and including first principle phonon-phonon scattering rates as reported recently for pristine graphene, reveals spectacular effects even for extremely low vacancy concentrations.

cond-mat.mtrl-sci↗

arXiv2017

Absence of confinement in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices

The reduction of dimensionality is an efficient pathway to boost the performances of thermoelectric materials, it leads to the quantum confinement of the carriers and thus to large Seebeck coefficients (S) and it also suppresses the thermal conductivity by increasing the phonon scattering processes. However, quantum confinement in superlattices is not always easy to achieve and needs to be carefully validated. In the past decade, large values of S have been measured in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices (Nat. Mater. 6, 129 (2007) and Appl. Phys. Lett. 91, 192105 (2007)). In the $δ$-doped compound, the measured S was almost 6 times larger than that of the bulk material. This huge increase has been attributed to the two dimensional confinement of the carriers in the doped regions. In this work, we demonstrate that the experimental data can be well explained quantitatively within the scenario in which electrons are delocalized in both in-plane and growth directions, hence strongly suggesting that the confinement picture in these superlattices may be unlikely.

cond-mat.mtrl-sci↗

arXiv2017

Unified modelling of the thermoelectric properties in SrTiO3

Thermoelectric materials are opening a promising pathway to address energy conversion issues governed by a competition between thermal and electronic transport. Improving the efficiency is a difficult task, a challenge that requires new strategies to unearth optimized compounds. We present a theory of thermoelectric transport in electron doped SrTiO3, based on a realistic tight binding model that includes relevant scattering processes. We compare our calculations against a wide panel of experimental data, both bulk and thin films. We find a qualitative and quantitative agreement over both a wide range of temperatures and carrier concentrations, from light to heavily doped. Moreover, the results appear insensitive to the nature of the dopant La, B, Gd and Nb. Thus, the quantitative success found in the case of SrTiO3, reveals an efficient procedure to explore new routes to improve the thermoelectric properties in oxides.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic Property of Rutile Ti0.94A0.06O2 (A=Li, Mg, K) Compounds

Our purpose is to study role of ionic radii of non-magnetic dopants; Li+ (0.68 Å), Mg2+ (0.72 Å) and K+ (1.38 Å) on the magnetic property of rutile TiO2 compound. The Ti0.94A0.06O2 (A=Li, Mg, K) compounds have been synthesized via solid state route method at equilibrium. The structural analyses of X-ray diffraction pattern reveals that the doping of Li and Mg lead to Ti site substitution and K doping lead to core shell kind of structure. The magnetic property measurement by SQUID magnetometer indicate that all compounds exhibit weak paramagnetism with highest paramagnetic moment of ~ 0.3 MuB / ion for K doped compound but no long-range ferromagnetic ordering. We have discussed the observed magnetism in correlation with the nature of substitution.

cond-mat.mtrl-sci↗

arXiv2013

Non-magnetic doping induced magnetism in Li doped SnO2 nanoparticles

We address the possibility of non-magnetic doping induced magnetism, in Li doped SnO2 nano-particles. The compounds have been prepared by solid state route at equilibrium and were found to be crystallized in single rutile phase. The magnetization measurements have shown that Li-doping induces magnetism in SnO2 for a particular range of Li concentration. However, for other Li concentrations, including pure SnO2, the samples exhibit diamagnetism. To investigate the possible origin of the induced magnetism, we have studied the variation of the magnetization as a function of the average nano-particle radius. Possible scenarios for the appearance of magnetism in these compounds are discussed.

cond-mat.mtrl-sci↗

arXiv2011

Absence of Ferromagnetism in Mn-doped Tetragonal Zirconia

In a recent letter, it has been predicted within first principle studies that Mn-doped ZrO2 compounds could be good candidate for spintronics application because expected to exhibit ferromagnetism far beyond room temperature. Our purpose is to address this issue experimentally for Mn-doped tetragonal zirconia. We have prepared polycrystalline samples of Y0.15(Zr0.85-yMny)O2 (y=0, 0.05, 0.10, 0.15 & 0.20) by using standard solid state method at equilibrium. The obtained samples were carefully characterized by using x-ray diffraction, scanning electron microscopy, elemental color mapping, X-ray photoemission spectroscopy and magnetization measurements. From the detailed structural analyses, we have observed that the 5% Mn doped compound crystallized into two symmetries (dominating tetragonal & monoclinic), whereas higher Mn doped compounds are found to be in the tetragonal symmetry only. The spectral splitting of the Mn 3s core-level x-ray photoelectron spectra confirms that Mn ions are in the Mn3+ oxidation state and indicate a local magnetic moment of about 4.5 μB/Mn. Magnetic measurements showed that compounds up to 10% of Mn doping are paramagnetic with antiferromagnetic interactions. However, higher Mn doped compound exhibits local ferrimagnetic ordering. Thus, no ferromagnetism has been observed for all Mn-doped tetragonal ZrO2 samples.

cond-mat.mtrl-sci↗

arXiv2010

Is room temperature ferromagnetism possible in K-doped SnO2?

Ab initio studies have theoretically predicted room temperature ferromagnetism in crystalline SnO2, ZrO2 and TiO2 doped with non magnetic element from the 1A column as K and Na. Our purpose is to address experimentally the possibility of magnetism in both Sn1-xKxO2 and Sn1-xCaxO2 compounds. The samples have been prepared using equilibrium methods of standard solid state route. Our study has shown that both Sn1-xCaxO2 and Sn1-xKxO2 structure is thermodynamically unstable and leads to a phase separation, as shown by X-ray diffraction and detailed micro-structural analyses with high resolution transmission electron microscopy (TEM). In particular, the crystalline SnO2 grains are surrounded by K-based amorphous phase. In contrast to Ca: SnO2 samples we have obtained a magnetic phase in K: SnO2 ones, but no long range ferromagnetic order. The K: SnO2 samples exhibit a moments of the order of 0.2 μB/K /ion, in contrast to ab-initio calculations which predict 3μB, where K atoms are on the Sn crystallographic site. The apparent contradictions between our experiments and first principle studies are discussed.

cond-mat.mtrl-sci↗

arXiv2007

Relation of Curie temperature and conductivity: (Ga,Mn)As alloy as a case study

Experimental investigations of diluted magnetic semiconductors indicate a strong relation between Curie temperature and conductivity. Both quantities depend non trivially on the concentration of magnetic impurities, the carrier density, and the presence of compensating defects. We calculate both Curie temperature and conductivity of (Ga,Mn)As alloys in a selfconsistent manner based on the same first principles Hamiltonian in which the presence of compensating defects is taken into account. The effect of As-antisites and Mn-interstitials is determined separately and a good agreement between theory and experiment exists only in the case where the dominating mechanism of is due to the Mn-interstitials.

cond-mat.dis-nn↗