arXiv subjects

Carmine Autieri

Publications and source records attributed to Carmine Autieri.

At least 73 records · Page 4Linked to original sources

arXiv2023

Current driven insulator-to-metal transition without Mott breakdown in Ca$_2$RuO$_4$

The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as Ca$_2$RuO$_4$, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. This phenomenon has numerous potential applications in, e.g., neuromorphic computing. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-driven conductive phase arises with only a minor reorganisation of the Mott state. This can be explained by the co-existence of structurally different domains that emerge during the IMT. Electronic structure calculations show that the boundaries between domains of slightly different structure lead to a drastic reduction of the overall gap. This permits an increased conductivity, despite the persistent presence of the Mott state. This mechanism represents a paradigm shift in the understanding of IMTs, because it does not rely on the simultaneous presence of a metallic and an insulating phase, but rather on the combined effect of structurally inhomogeneous Mott phases.

cond-mat.str-el↗

arXiv2023

Sign change of the anomalous Hall effect and the anomalous Nernst effect in Weyl semimetal CeAlSi

We report the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) data for the non-collinear Weyl semimetal CeAlSi. The anomalous Hall conductivity (σ_ij^A) was measured for two different orientations of the magnetic field (B), namely σ_yz^A for B II a and σ_xy^A for B II c, where a and c denote the crystallographic axes. We find that σ_xy^A and σ_yz^A are of opposite sign and both are large below the Curie temperature (T_C). In the paramagnetic phase, σ_xy^A raises even more and goes through a maximum at T ~ 170 K, whereas the absolute value of σ_yz^A decreases with increasing temperature. The origin of the sign difference between σ_xy^A and σ_yz^A was attributed to the reconstruction of the band structure under the variation of the spin orientation. Further, in a system where humps in the AHE are present and scalar spin chirality is zero, we show that the k-space topology plays an important role to determine the transport properties at both low and high temperatures. We also observed the anomalous contribution in the Nernst conductivity (α_xy^A) measured for B II c. α_xy^A/T turns out to be sizeable in the magnetic phase and above T_C slowly decreases with temperature. We were able to recreate the temperature dependences of σ_xy^A and α_xy^A/T in the paramagnetic phase using a single band toy-model assuming a non-zero Berry curvature in the vicinity of the Weyl node. A decisive factor appears to be a small energy distance between the Fermi level and a Weyl point.

cond-mat.str-el↗

arXiv2023

Engineering axion insulator phase in superlattices with inversion symmetry breaking

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane Néel vector direction below a critical thickness of MnTe, which is the ground state amongst all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic two-fold rotational symmetry $C_2{\cdot}T$. The axion insulator phase evolves into a trivial insulator as we increase the thickness of the magnetic layers. By switching the Néel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the Néel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice.

cond-mat.mtrl-sci↗

arXiv2023

Correlation-driven topological transition in Janus VSiGeP2As2

The appearance of intrinsic ferromagnetism in 2D materials opens the possibility of investigating the interplay between magnetism and topology. The magnetic anisotropy energy (MAE) describing the easy axis for magnetization in a particular direction is an important yardstick for nanoscale applications. Here, the first-principles approach is used to investigate the electronic band structures, the strain dependence of MAE in pristine VSi2Z4 (Z=P, As) and its Janus phase VSiGeP2As2 and the evolution of the topology as a function of the Coulomb interaction. In the Janus phase the compound presents a breaking of the mirror symmetry, which is equivalent to having an electric field, and the system can be piezoelectric. It is revealed that all three monolayers exhibit ferromagnetic ground state ordering, which is robust even under biaxial strains. A large value of coupling J is obtained, and this, together with the magnetocrystalline anisotropy, will produce a large critical temperature. We found an out-of-plane (in-plane) magnetization for VSi2P4 (VSi2As4), while in-plane magnetization for VSiGeP2As2. Furthermore, we observed a correlation-driven topological transition in the Janus VSiGeP2As2. Our analysis of these emerging pristine and Janus-phased magnetic semiconductors opens prospects for studying the interplay between magnetism and topology in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2023

Pressure-induced dimerization and molecular orbitals formation in Na2RuO3 with strong correlation-enhanced spin-orbit coupling effect

First-principles calculations and simulations are conducted to clarify the nonmagnetic insulating ground state of the honeycomb lattice compound Na2RuO3 with 4d^4 electronic configuration and explore the evolutions of crystal structure and electronic property under pressure. We reveal that individual Coulomb correlation or spin-orbit coupling (SOC) effect cannot reproduce the experimentally observed nonmagnetic insulating behavior of Na2RuO3, whereas the Coulomb correlation enhanced SOC interactions give rise to an unusual spin-orbital-entangled J = 0 nonmagnetic insulating state, which contrasts with the SOC assisted Mott insulating state in d^5 ruthenates and iridates. Furthermore, a pressure-induced structural dimerization transition has been predicted around 15-17.5 GPa. The honeycomb lattice of the high-pressure dimerized phase features with parallel pattern of the short Ru-Ru dimers aligning along the crystallographic b direction. Accompanied with the structural dimerization, the electronic structure shows striking reconstruction by formation of molecular orbitals. Interestingly, the cooperation of Coulomb correlation together with SOC can realize a nonmagnetic insulating state in the high-pressure dimerized phase. The d^4 ruthenate Na2RuO3 with honeycomb lattice will provide a new platform to explore unusual physics and rich phase diagram due to the delicate interplay of lattice degree of freedom, electron correlations, and SOC interactions.

cond-mat.str-el↗

arXiv2023

Interfacial Dzyaloshinskii-Moriya interaction in epitaxial W/Co/Pt multilayers

Dzyaloshinskii-Moriya interaction (DMI) manifesting in asymmetric layered ferromagnetic films gives rise to non-colinear spin structures stabilizing magnetization configurations with nontrivial topology. In this work magnetization reversal, domain structure, and strength of DMI are related with the structure of W/Co/Pt multilayers grown by molecular beam epitaxy. Applied growth method enables fabrication of layered systems with higher crystalline quality than commonly applied sputtering techniques. As a result, a high value of D coefficient was determined from the aligned magnetic domain stripe structure, substantially exceeding 2 mJ/m2. The highest value of DMI value D$_{eff}$ = 2.64mj/m2 and strength of surface DMI parameter DS = 1.83pJ/m for N=10 has been observed. Experimental results coincide precisely with those obtained from structure based micromagnetic modelling and density functional theory calculations performed for well-defined layered stacks. This high value of DMI strength originates from dominating contributions of the interfacial atomic Co layers and additive character from both interface types.

cond-mat.mtrl-sci↗

arXiv2023

Topological transition in Pb1-xSnxSe using Meta-GGA

We calculate the mirror Chern number (MCN) and the band gap for the alloy Pb1-xSnxSe as a function of the concentration x by using virtual crystalline approximation. We use the electronic structure from the relativistic density functional theory calculations in the Generalized-Gradient- Approximation (GGA) and meta-GGA approximation. Using the modified Becke-Johnson meta- GGA functional, our results are comparable with the available experimental data for the MCN as well as for the band gap. We advise to use modified Becke-Johnson approximation with the parameter c=1.10 to describe the transition from trivial to topological phase for this class of compounds.

cond-mat.mtrl-sci↗

arXiv2023

Topological phase diagram of Pb1-xSnxSe1-yTey

We reproduce the mirror Chern number phase diagram for the quaternary compound Pb1-xSnxSe1-yTey combining accurate density functional theory and tight-biding model. The tight binding models are extracted from ab-initio results, adding constraints to reproduce the experimental results. By using the virtual crystalline approximation, we calculated the mirror Chern number as a function of the concentrations x and y. We report different hamiltonians depending on whether we want to focus on the experimental band gap or on the experimental topological transition. We calculate the transition line between the trivial insulating phase and topological insulating phase that results in good agreement with the experimental results. Finally, we add in the phase diagram the Weyl phase predicted in the literature providing a complete topological phase diagram for the Pb1-xSnxSe1-yTey quaternary compound.

cond-mat.mtrl-sci↗

arXiv2023

Mott-Insulator Ca2RuO4 under a static external electric field

We have investigated the structural, electronic and magnetic properties of the Mott-insulator Ca2RuO4 under the application of a static external electric field in two regimes: bulk systems at small fields and thin films at large electric fields. Ca2RuO4 presents an S- and L-Pbca phase with short and long c lattice constants and with large and small band gaps, respectively. Using density functional perturbation theory, we have calculated the Born effective charges as response functions. Once we break the inversion symmetry by off-centering the Ru atoms, we calculate the piezoelectric properties of the system that suggest an elongation of the system under an electric field. Finally, we investigated a four unit cells slab at larger electric fields and we found insulator-metal transitions induced by the electric field. By looking at the local density of states, we have found that the gap gets closed on surface layers while the rest of the sample is insulating. Correlated to the electric-filed-driven gap closure, there is an increase in the lattice constant c. Regarding the magnetic properties, we have identified two phase transitions in the magnetic moments with one surface that gets completely demagnetized at the largest field investigated. In all cases, the static electric field increases the lattice constant c and reduces the band gap of Ca2RuO4 playing a role in the competition between the L-phase and the S-phase.

cond-mat.str-el↗

arXiv2022

In-gap states and strain-tuned band convergence in layered structure trivalent iridate K0.75Na0.25IrO2

Iridium oxides (iridates) provide a good platform to study the delicate interplay between spin-orbit coupling (SOC) interactions, electron correlation effects, Hund's coupling and lattice degree of freedom. However, overwhelming investigations primarily focus on tetravalent (Ir4+, 5d5) and pentavalent (Ir5+, 5d4) iridates, far less attention has been paid to iridates with other valence states. Here, we pay our attention to a less-explored trivalent (Ir3+, 5d6) iridates, K0.75Na0.25IrO2, crystalizing in a triangular lattice with edge-sharing IrO6 octahedra and alkali metal ions intercalated [IrO2]- layers. We theoretically determine the preferred occupied positions of the alkali metal ions from energetic viewpoints and reproduce the experimentally observed semiconducting behavior and nonmagnetic (NM) properties. The SOC interactions play a critical role in the band dispersion, resulting in NM Jeff = 0 states. More intriguingly, our electronic structure not only uncovers the presence of in-gap states and explains the abnormal low activation energy in K0.75Na0.25IrO2, but also predicts the band edge can be effectively modulated by mechanical strain. Especially, the in-gap states feature with enhanced band-convergence characteristics by 6% compressive strain, which will greatly enhance the electrical conductivity of K0.75Na0.25IrO2. Present work sheds new lights on the unconventional electronic structures of the trivalent iridates, indicating its promising application as nanoelectronic and thermoelectric material.

cond-mat.str-el↗

arXiv2022

Unprotected edge modes in quantum spin Hall insulator candidate materials

The experiments in quantum spin Hall insulator candidate materials, such as HgTe/CdTe and InAs/GaSb heterostructures, indicate that in addition to the topologically protected helical edge modes these multilayer heterostructures may also support additional edge states, which can contribute to the scattering and the transport. We use first-principles calculations to derive an effective tight-binding model for HgTe/CdTe, HgS/CdTe and InAs/GaSb heterostructures, and we show that all these materials support additional edge states which are sensitive to the edge termination. We trace the microscopic origin of these states back to a minimal model supporting flat bands with a nontrivial quantum geometry that gives rise to polarization charges at the edges. We show that the polarization charges transform into the additional edge states when the flat bands are coupled to each other and to the other states to form the Hamiltonian describing the full heterostructure. Interestingly, in the HgTe/CdTe quantum wells the additional edge states are far away from the Fermi level so that they do not contribute to the transport but in the HgS/CdTe and InAs/GaSb heterostructures they appear within the bulk energy gap giving rise to the possibility of multimode edge transport. Finally, we demonstrate that because these additional edge modes are non-topological it is possible to remove them from the bulk energy gap by modifying the edge potential for example with the help of a side gate or chemical doping.

cond-mat.mes-hall↗

arXiv2022

Switchable large-gap quantum spin Hall state in two-dimensional MSi$_2$Z$_4$ materials class

Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically protected from backscattering and offer unique opportunities for addressing fundamental science questions and device applications. Finding viable materials that host such topological states, however, remains a challenge. Here by using in-depth first-principles theoretical modeling, we predict large bandgap QSH insulators in recently bottom-up synthesized two-dimensional (2D) MSi$_2$Z$_4$ (M = Mo or W and Z = P or As) materials family with $1T^\prime$ structure. A structural distortion in the $2H$ phase drives a band inversion between the metal (Mo/W) $d$ and $p$ states of P/As to realize spinless Dirac cone states without spin-orbit coupling. When spin-orbit coupling is included, a hybridization gap as large as $\sim 204$ meV opens up at the band crossing points, realizing spin-polarized conducting edge states with nearly quantized spin Hall conductivity. We also show that the inverted band gap is tunable with a vertical electric field which drives a topological phase transition from the QSH to a trivial insulator with Rashba-like edge states. Our study identifies 2D MSi$_2$Z$_4$ materials family with $1T^\prime$ structure as large bandgap, tunable QSH insulators with protected spin-polarized edge states and large spin-Hall conductivity.

cond-mat.mes-hall↗

arXiv2022

Topological Lifshitz transition in Weyl semimetal NbP decorated with heavy elements

Studies of the Fermi surface modification after in-situ covering NbP semimetal with heavy elements Pb and Nb ultrathin layers were performed by means of angle-resolved photoemission spectroscopy (ARPES). First, the electronic structure was investigated for pristine single crystals with two possible terminations (P and Nb) of the (0 0 1) surface. The nature of the electronic states of these two cleaving planes is different: P-terminated surface shows spoon and bow tie shaped fingerprints, whereas these shapes are not present in Nb-terminated surfaces. ARPES studies show that even 1 monolayer (ML) of Pb causes topological quantum Lifshitz transition (TQLT) in P- and Nb-terminated surfaces. Deposited Pb 5d electrons have wide extended atomic orbitals which leads to strong hybridization with Pb-terminated surface and a corresponding shift in the Fermi energy. Nb has less capability to perturb the system than Pb because Nb has weaker spin-orbit coupling than Pb. Nb-terminated surface subjected to surface decoration with approximately 1.3 ML of Nb shows no dramatic modification in the Fermi surface. In the case of Nb decorated P-terminated surface, deposition of approximately 1 ML modifies the electronic structure of NbP and it is on the verge of TQLT. Despite the strong spin-orbit and strong hybridization of the heavy elements on the surface, it is possible to observe the TQLT of the surface states thanks to the robustness of the bulk topology.

cond-mat.mtrl-sci↗

arXiv2022

Emergence of Rashba splitting and spin-valley properties in Janus MoGeSiP2As2 and WGeSiP2As2 monolayers

First-principles calculations are performed to study the structural stability and spintronics properties of Janus MoGeSiP2As2 and WGeSiP2As2 monolayers. The high cohesive energies and the stable phonon modes confirm that both these structures are experimentally accessible. In contrast to pristine MoSi2P4, the Janus monolayers demonstrate reduced direct bandgaps and large spin-split states at K/-K. In addition, their spin textures exposed that breaking the mirror symmetry brings Rashba-type spin splitting in the systems which can be increased by using higher atomic spin-orbit coupling. The large valley spin splitting together with the Rashba splitting in these Janus monolayer structures can make a remarkable contribution to semiconductor valleytronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2022

Strain Modulated Electronic and Optical Properties of Laterally Stitched MoSi2N4/XSi2N4 (X=W, Ti) 2D Heterostructures

We used first-principles calculations to investigate the laterally stitched monolayered MoSi2N4/XSi2N4 (X=W, Ti) 2D heterostructures. The structural stability of such heterostructures is confirmed by the phonon spectra exhibiting no negative frequencies. From the electronic band structures, the MoSi2N4/WSi2N4-lateral heterostructure (MWLH) shows semiconducting nature with an indirect bandgap of 2.35 eV, while the MoSi2N4/TiSi2N4-lateral heterostructure (MTLH) revealed metallic behavior. Moreover, the effect of biaxial strain on the electronic and optical properties of MWLH is studied, which indicated substantial modifications in their electronic and optical spectra. In particular, an indirect to direct bandgap semiconducting transition can be achieved in MWLH via compressive strain. Besides, the absorbance, transmittance and reflectance spectra can effectively be tuned by means of biaxial strain. Our findings provide insights into the strain engineering of electronic and optical features, which could pave the way for future nano- and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2022

Topological states in superlattices of HgTe-class materials for engineering three-dimensional flat bands

In search of materials with three-dimensional flat band dispersions, using {\em ab-initio} computations, we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion.

cond-mat.mtrl-sci↗

arXiv2022

Electronic and optical properties of InAs/InAs$_{0.625}$Sb$_{0.375}$ superlattices and their application to far-infrared detectors

We calculate the electronic and optical properties of InAs/InAs$_{0.625}$Sb$_{0.375}$ superlattices within relativistic density functional theory. To have a good description of the electronic and optical properties, the modified Becke-Johnson exchange-correlation functional is pondered to correctly approximate the band gap. First, we analyze electronic and optical characteristics of bulk InAs and InSb, and then we investigate the InAs/InAs$_{0.625}$Sb$_{0.375}$ superlattice. The optical gaps deduced from the imaginary part of the dielectric function are associated with the characteristic interband transitions. We investigate the electronic and optical properties of the InAs/InAs$_{0.625}$Sb$_{0.375}$ superlattice with three lattice constants of the bulk InAs, GaSb and AlSb, respectively. It is observed that the electronic and optical properties strongly depend on the lattice constant. Our results support the presence of two heavy-hole bands with increasing in-plane effective mass as we go far from the Fermi level. We notice a considerable decrease in the energy gaps and the effective masses of the heavy-holes in the k$_x$-k$_y$ plane compared to the bulk phases of the parent compounds. We demonstrate that the electrons are s-orbitals delocalized in the entire superlattice, while the holes have mainly 5p-Sb character localized in the In(As,Sb) side of the superlattice. In the superlattice, the low-frequency absorption spectra greatly increase when the electric field is polarized orthogonal to the growth axis allowing the applicability of III-V compounds for the long-wavelength infrared detectors.

cond-mat.mtrl-sci↗

arXiv2022

Limited ferromagnetic interactions in monolayers of MPS$_3$ (M=Mn, Ni)

We present a systematic study of the electronic and magnetic properties of two-dimensional ordered alloys, consisting of two representative hosts (MnPS$_3$ and NiPS$_3$) of transition metal phosphorus trichalcogenides doped with $3d$ elements. For both hosts our DFT+U calculations are able to qualitatively reproduce the ratios and signs of all experimentally observed magnetic couplings. The relative strength of all antiferromagnetic exchange couplings, both in MnPS$_3$ as well as in NiPS$_3$, can successfully be explained using an effective direct exchange model: they reveal that the third-neighbor exchange dominates in NiPS$_3$ due to the filling of the $t_{2g}$ subshell, whereas for MnPS$_3$ the first neighbor exchange is prevailed owing to the presence of the $t_{2g}$ magnetism. On the other hand, the nearest neighbor ferromagnetic coupling in NiPS$_3$ can only be explained using a more complex superexchange model and is (also) largely triggered by the absence of the $t_{2g}$ magnetism. For the doped systems, the DFT+U calculations revealed that magnetic impurities do not affect the magnetic ordering observed in the pure phases and thus in general in these systems ferromagnetism may not be easily induced by such a kind of elemental doping. However, unlike for the hosts, the first and second (dopant-host) exchange couplings are of similar order of magnitude. This leads to frustration in case of antiferromagnetic coupling and may be one of the reasons of the observed lower magnetic ordering temperature of the doped systems.

cond-mat.mtrl-sci↗