arXiv subjects

Ilaria Pallecchi

Publications and source records attributed to Ilaria Pallecchi.

16 recordsLinked to original sources

arXiv2026

Best practices for a proper evaluation and conversion of physical property equations in superconductors: the examples of WHH formulation, Bean model and other cases of interest

In recent years there have been growing concerns about the proper evaluation of physical properties of superconductors, in particular for quantities extracted from magnetic characterizations. Errors can and often do occur due to the following issues: i) several measurement instruments still use Gaussian & cgs-emu units instead of the preferable International System (SI) units; ii) there are decades of valuable publications where, however, equations were expressed in Gaussian & cgs-emu or other unit systems or where constants were normalized to unity, which requires proper understanding and unit conversion in order to correctly evaluate the measured physical quantities; iii) the conversion between unit systems sometimes appears challenging and may not be properly performed. In this paper we will describe how to properly convert physical quantities relevant for the evaluation of magnetic and other properties focusing on the still most used unit systems, SI and Gaussian & cgs-emu. We will provide examples of how to properly verify and understand the physical formulae. We will include examples for the correct method to determine the critical current density Jc of a superconductor from the measurement of its magnetic hysteresis loop through the Bean model, and the correct conversion to SI of the equations for Hc2(T) according to the Werthamer-Helfand-Hohenberg (WHH) formulation. The goals of this paper are to make the readers aware of the unit conversion issue, to provide useful hands-on tools for proper conversion and to strongly encourage future exclusive use of the SI units and formulae.

cond-mat.supr-con↗

arXiv2025

Inhomogeneity-driven multiform Spontaneous Hall Effect in conventional and unconventional superconductors

The spontaneous Hall effect (SHE), a finite voltage occurring transversal to the electrical current in zero-magnetic field, has been observed in both conventional and unconventional superconductors, appearing as a peak near the superconducting transition temperature. The origin of SHE is strongly debated, with proposed explanations ranging from intrinsic and extrinsic mechanisms such as spontaneous symmetry breaking and time-reversal symmetry breaking (BTRS), Abrikosov vortex motion, or extrinsic factors like material inhomogeneities, such as non-uniform critical temperature (Tc) distributions or structural asymmetries. This work is an experimental study of the SHE in various superconducting materials. We focused on conventional, low-Tc, sharp transition Nb and unconventional, intermediate-Tc, smeared transition Fe(Se,Te). Our findings show distinct SHE peaks around the superconducting transition, with variations in height, sign and shape, indicating a possible common mechanism independent of the specific material. We propose that spatial inhomogeneities in the critical temperature, caused by local chemical composition variations, disorder, or other forms of electronic spatial inhomogeneities could explain the appearing of the SHE. This hypothesis is supported by comprehensive finite elements simulations of randomly distributed Tc by varying Tc-distribution, spatial scale of disorder and amplitude of the superconducting transition. The comparison between experimental results and simulations suggest a unified origin for the SHE in different superconductors, whereas different phenomenology can be explained in terms of amplitude of the transition temperature in respect to Tc-distribution.

cond-mat.supr-con↗

arXiv2025

Highly efficient field-free switching by orbital Hall torque in a MoS2-based device operating at room temperature

Charge-to-spin and spin-to-charge conversion mechanisms in high spin-orbit materials are the new frontier of memory devices. They operate via spin-orbit torque (SOT) switching of a magnetic electrode, driven by an applied charge current. In this work, we propose a novel memory device based on the semiconducting two-dimensional centrosymmetric transition metal dichalcogenide (TMD) MoS2, that operates as a SOT device in the writing process and a spin valve in the reading process. We demonstrate that stable voltage states at room temperature can be deterministically controlled by a switching current density as low as 3.2x10^4 A/cm^2 even in zero field, owed to a tilted geometry and a differential voltage architecture. An applied field of 50-100 Oe can be used as a characterizing control parameter for the state switching. Ab initio calculations of spin Hall effect (SHE) and orbital Hall effect (OHE) point to the latter as the most likely responsible for the generation of the SOT in the magnetic electrode. The large value of OHC in bulk MoS2 makes our device competitive in terms of energetic efficiency and could be integrated in TMD heterostructures to design memory devices with multiple magnetization states for non-Boolean computation.

cond-mat.mes-hall↗

arXiv2025

Roles of Defects and Sb-doping in the Thermoelectric Properties of Full-Heusler Fe2TiSn

The potential of Fe2TiSn full-Heusler compounds for thermoelectric applications has been suggested theoretically, but not yet grounded experimentally, due to the difficulty of obtaining reproducible, homogeneous, phase pure and defect free samples. In this work, we study Fe2TiSn1-xSbx polycrystals (x from 0 to 0.6), fabricated by high-frequency melting and long-time high-temperature annealing. We obtain fairly good phase purity, homogeneous microstructure and good matrix stoichiometry. Although intrinsic p-type transport behavior is dominant, n-type charge compensation by Sb doping is demonstrated. Calculations of formation energy of defects and electronic properties carried out in the density functional theory formalism reveal that charged iron vacancies VFe2- are the dominant defects responsible for the intrinsic p-type doping of Fe2TiSn in all types of growing conditions except Fe-rich. Additionally, Sb substitutions at Sn site give rise either to SbSn, SbSn1+ which are responsible for n-type doping and magnetism (SbSn) or to magnetic SbSn1- which act as additional p-type dopants. Our experimental data highlight good thermoelectric properties close to room temperature, with Seebeck coefficients up to 56 microV/K in the x=0.2 sample and power factors up to 4.8x10^-4 W m^-1 K^-2 in the x=0.1 sample. Our calculations indicate the appearance of a pseudogap in Ti-rich conditions and large Sb doping, possibly improving further the thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2025

Irreversible multi-band effects and Lifshitz transitions at the LaAlO3/SrTiO3 interface under field effect

In this work, we investigate the irreversible effects of an applied electric field on the magnetotransport properties of LaAlO3/SrTiO3 conducting interfaces, with focus on their multiband character. We study samples of different types, namely with either crystalline or amorphous LaAlO3 overlayer. Our two-band analysis highlights the similarity of the electronic properties of crystalline and amorphous interfaces, regardless much different carrier densities and mobilities. Furthermore, filling and depletion of the two bands follow very similar patterns, at least in qualitative terms, in the two types of samples. In agreement with previous works on crystalline interfaces, we observe that an irreversible charge depletion takes place after application of a first positive back gate voltage step. Such charge depletion affects much more, in relative terms, the higher and three-dimensional dyz, dzx bands than the lower and bidimensional dxy, driving the system through the Lifshitz transition from two-band to single band behavior. The quantitative analysis of experimental data evidences the roles of disorder, apparent in the depletion regime, and temperature. Noteworthy, filling and depletion of the two bands follow very similar patterns in crystalline and amorphous samples, at least in qualitative terms, regardless much different carrier densities and mobilities.

cond-mat.mtrl-sci↗

arXiv2025

The uncollapsed LaFe2As2 phase: compensated, highly doped, electron-phonon coupled, iron-based superconductor

The recently discovered LaFe2As2 superconducting compound, member of the 122 family of iron pnictide superconductors, becomes superconducting below Tc=13K, yet its nominal doping apparently places it in the extreme overdoped limit, where superconductivity should be suppressed. In this work, we investigate the normal state of magneto- and thermo-electric transport and specific heat of this compound. The experimental data are consistent with the presence of highly compensated electron and hole bands, with around 0.42 electrons per unit cell just above Tc, and high effective masses around 3m0. The temperature dependence of transport properties strongly resembles that of conventional superconductors, pointing to a key role of electron-phonon coupling. From these evidences, LaFe2As2 can be regarded as the connecting compound between unconventional and conventional superconductors.

cond-mat.supr-con↗

arXiv2024

Evolution of dissipative regimes in atomically thin $\text{Bi}_{2}\text{Sr}_{2}\text{CaCu}_{2}\text{O}_{8+x}$ superconductor

Thermoelectric transport has been widely used to study Abrikosov vortex dynamics in unconventional superconductors. However, only a few thermoelectric studies have been conducted near the dimensional crossover that occurs when the vortex-vortex interaction length scale becomes comparable to the sample size. Here we report the effects of finite size on the dissipation mechanisms of the Nernst effect in the optimally doped $\text{Bi}_{2}\text{Sr}_{2}\text{CaCu}_{2}\text{O}_{8+x}$ high-temperature superconductor, down to the atomic length limit. To access this regime, we develop a new generation of thermoelectric chips based on silicon nitride microprinted circuit boards. These chips ensure optimized signals while preventing sample deterioration. Our results demonstrate that lateral confinement at the nanoscale can effectively reduce vortex dissipation. Investigating vortex dissipation at the micro- and nano-scale is essential for creating stable, miniaturized superconducting circuits.

cond-mat.supr-con↗

arXiv2023

Experimental investigation of electronic interactions in collapsed and uncollapsed LaFe2As2 phases

The iron-based pnictide LaFe2As2 is not superconducting as-synthesized, but it becomes such below Tc around 12 K upon annealing, as a consequence of a structural transition from a phase with collapsed tetragonal crystal structure to an uncollapsed phase. In this work, we carry out specific heat, Raman spectroscopy and normal state electric and thermoelectric transport measurements in the collapsed and uncollapsed LaFe2As2 phases to gain insight into the electron interactions and their possible role in the superconducting pairing mechanism. Despite clear features of strong electron-phonon coupling observed in both phases, neither the low energy phonon spectra nor the electron-phonon coupling show significant differences between the two phases. Conversely, the Sommerfield constants are significantly different in the two phases, pointing to much higher electron correlation in the superconducting uncollapsed phase and confirming theoretical studies.

cond-mat.supr-con↗

arXiv2019

Band filling and disorder effects on the normal state thermoelectric behavior in MgB2

By a combined experimental and theoretical approach, we investigate normal state thermoelectric transport in MgB2, as a probe of selective disorder and doping in the sigma and pi bands. We calculate the temperature dependent diffusive Seebeck coefficient Sdiff(T) with the Boltzmann equation resolved in relaxation time approximation, taking into account the scattering with phonons and impurities, the effect of renormalization and the effect doping in a rigid band approximation. We show that selective disorder has a sizeable effect on the Sdiff magnitude, as it tunes the relative contributions of sigma and pi bands. Disorder also affects the Sdiff temperature dependences, eventually yielding a linear Sdiff(T) behavior in the dirty limit. We also show that band filling has opposite effects on S, depending on which band dominates transport. In parallel, we carry out Seebeck effect measurements on neutron-irradiated Mg11B2, and on two series of doped samples Mg1-xAlxB2 and Mg(B1-xCx)2. From comparison of calculated Sdiff(T) and experimental S(T) curves, we demonstrate that diffusive and phonon drag terms give comparable contributions in clean samples, but the phonon drag term is progressively suppressed with increasing disorder. In C and Al doped samples we observe very different experimental behaviors in terms of sign, magnitude and temperature dependence. Indeed, notwithstanding the similar electron doping introduced by both substitutions, C or Al doping yields disorder which mainly affects either sigma or pi bands, respectively. With the help of our ab-initio approach, we are able to disentangle the several effects and prove that Seebeck coefficient is a very sensitive probe of the kind of disorder.

cond-mat.supr-con↗

arXiv2018

Evidence of the isoelectronic character of F doping in SmFeAsO(1-x)F(x): a first-principles investigation

We study the electronic structure of the SmFeAsO(1-x)F(x) alloy by means of first-principle calculations. We find that, contrary to common believe, F-doping does not change the charge balance between electrons and holes free-carriers in SmFeAsO(1-x)F(x). For energies within a narrow energy range across E_F, the effect of F-doping on the band structure dispersion is tiny in both the paramagnetic and stripe antiferromagnetic phase. The charge balance between the conducting FeAs-layer and the SmO(1-x)F(x) charge reservoir layer is not influenced by the compositional change. The additional charge carried by fluorine, with respect to the oxygen, is compensated by a change in the oxidation state of the Sm ion from 3+ to 2+. A comparison with the SmFe(1-x)Co(x)AsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site.

cond-mat.supr-con↗

arXiv2018

Balanced electron-hole transport in spin-orbit semimetal SrIrO3 heterostructures

Relating the band structure of correlated semimetals to their transport properties is a complex and often open issue. The partial occupation of numerous electron and hole bands can result in properties that are seemingly in contrast with one another, complicating the extraction of the transport coefficients of different bands. The 5d oxide SrIrO3 hosts parabolic bands of heavy holes and light electrons in gapped Dirac cones due to the interplay between electron-electron interactions and spin-orbit coupling. We present a multifold approach relying on different experimental techniques and theoretical calculations to disentangle its complex electronic properties. By combining magnetotransport and thermoelectric measurements in a field-effect geometry with first-principles calculations, we quantitatively determine the transport coefficients of different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients, yielding a holelike response under field-effect and thermoelectric measurements and a linear, electronlike Hall effect up to 33 T.

cond-mat.str-el↗

arXiv2017

Effects of high energy proton irradiation on the superconducting properties of Fe(Se,Te) thin films

In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF2. In particular, we carry out a systematic experimental investigation with different irradiation fluences up to 7.30x10^16 cm^-2 and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, the resistivity and the critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small as compared to the case of other iron-based superconductors. Such effect is more evident on the critical current density Jc, while it is minor on the transition temperature Tc, on the normal state resistivity and on the upper critical field Hc2 up to the highest fluences explored in this work. In addition, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K, while there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film-substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and the lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators.

cond-mat.supr-con↗

arXiv2016

Nanopatterning process based on epitaxial masking for the fabrication of electronic and spintronic devices made of La0.67Sr0.33MnO3/LaAlO3/SrTiO3 heterostructures with in situ interfaces

The fabrication of oxide electronics devices is presently hindered by the lack of standardized and well established patterning procedures, applicable down to the nanoscale. In this work, the authors propose a procedure to obtain patterns with resolution around 100 nm on (La,Sr)MnO3/LaAlO3/SrTiO3 heterostructures. Our method is based on a multistep technique, which includes wet and dry etching, epitaxial masking, and e-beam lithography. Our procedure is devised to define independent patterns on the interfacial two dimensional electron gas and on the metallic top electrode, while preserving an all-in situ approach for the heterostructure growth. The authors show results on nano-scale devices based on (La,Sr)MnO3/LaAlO3/SrTiO3, suitable for oxide spintronics applications.

cond-mat.mes-hall↗

arXiv2014

Persistent photoconductivity in 2-dimensional electron gases at different oxide interfaces

We report on the transport characterization in dark and under light irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non- polar character and a critical thickness of four unit cells for the onset of conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3 are probed by atomic-scale- resolved electron energy loss spectroscopy showing that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface. Electric transport measurements performed in dark and under radiation show remarkable similarities and provide evidence that the persistent perturbation induced by light is an intrinsic peculiar property of the three investigated oxide-based polar/non-polar interfaces. Our work sets a framework for understanding the previous contrasting results found in literature about photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the origin of persistent photoconductivity and the origin of conductivity itself. An improved understanding of the photo- induced metastable electron-hole pairs might allow to shed a direct light on the complex physics of this system and on the recently proposed perspectives of oxide interfaces for solar energy conversion.

cond-mat.str-el↗

arXiv2012

Reversible and Persistent Photoconductivity at the NdGaO3/SrTiO3 Conducting Interface

The interface between the band gap insulators LaAlO3 and SrTiO3 is known to host a highly mobile two-dimensional electron gas. Here we report on the fabrication and characterization of the NdGaO3/SrTiO3 interface, that shares with LaAlO3/SrTiO3 an all-perovskite structure, the insulating nature of the single building block and the polar-non polar character. Our work demonstrates that in NdGaO3/SrTiO3 a metallic layer of mobile electrons is formed, with properties comparable to LaAlO3/SrTiO3. The localization of the injected electrons at the Ti sites, within a few unit cells from the interface, was proved by Atomic-scale-resolved EELS analyses. The electric transport and photoconduction of samples were also investigated. We found that irradiation by photons below the SrTiO3 gap does not increase the carrier density, but slightly enhances low temperature mobility. A giant persistent photoconductivity effect was instead observed, even under irradiation by low energy photons, in highly resistive samples fabricated at non-optimal conditions. We discuss the results in the light of different mechanisms proposed for the two-dimensional electron gas formation. Both the ordinary and the persistent photoconductivity in these systems are addressed and analyzed.

cond-mat.mtrl-sci↗

arXiv2010

Conducting interfaces between band insulating oxides: the LaGaO3/SrTiO3

We show that the growth of the heterostructure LaGaO3/SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3/SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-non polar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.

cond-mat.mtrl-sci↗