arXiv subjects

Giuseppe Cuono

Publications and source records attributed to Giuseppe Cuono.

49 records · Page 3Linked to original sources

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↗

arXiv2022

Dimensionality of the superconductivity in the transition metal pnictide WP

We report theoretical and experimental results on the transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP is a three-dimensional superconductor with an anisotropic electronic structure and nonsymmorphic symmetries. On the other hand, magnetoresistance experimental data and the analysis of superconducting fluctuations of the conductivity in external magnetic field indicate a weakly anisotropic three-dimensional superconducting phase.

cond-mat.supr-con↗

arXiv2021

Double-helix magnetic order in CrAs with Pnma space group

The transition metal pnictide CrAs exhibits superconductivity in the vicinity of a helimagnetic phase, where it has been found that the propagation vector is parallel to the c axis and the magnetic moments lie in the ab plane. Here we use ab initio calculations to study the magnetic interactions in the material. Mapping onto a Heisenberg Hamiltonian, we calculate the magnetic exchanges by using LDA+U calculations and we unveil the origin of the magnetic frustration. Finally, we reproduce the double helix magnetic order with a propagation vector Q = (0, 0, 0.456) and we obtain the magnetic transition temperature TN through Monte-Carlo simulations of the specific heat. Due to the limitations of the use of the Heisenberg Hamiltonian for itinerant magnetic systems, the theoretical TN underestimated the experimental value of the pure CrAs. However, our results are in good agreement with those found for the alloy CrAs0.5Sb0.5 belonging to the same space group, showing that our result can describe this material class.

cond-mat.supr-con↗

arXiv2021

Momentum-resolved spin splitting in Mn-doped trivial CdTe and topological HgTe semiconductors

Exchange coupling between localized spins and band or topological states accounts for giant magnetotransport and magnetooptical effects as well as determines spin-spin interactions in magnetic insulators and semiconductors. However, even in archetypical dilute magnetic semiconductors such as Cd$_{1-x}$Mn$_x$Te and Hg$_{1-x}$Mn$_x$Te the evolution of this coupling with the wave vector is not understood. A series of experiments have demonstrated that exchange-induced splitting of magnetooptical spectra of Cd$_{1-x}$Mn$_x$Te and Zn$_{1-x}$Mn$_x$Te at the L points of the Brillouin zone is, in contradiction to the existing theories, more than one order of magnitude smaller compared to its value at the zone center and can show an unexpected sign of the effective Landé factors. The origin of these findings we elucidate quantitatively by combining: (i) relativistic first-principles density functional calculations; (ii) a tight-binding approach that takes carefully into account k-dependence of the potential and kinetic sp-d exchange interactions; (iii) a theory of magnetic circular dichroism (MCD) for $E_1$ and $E_1$ + $Δ_1$ optical transitions, developed here within the envelope function $kp$ formalism for the L point of the Brillouin zone in zinc-blende crystals. This combination of methods leads to the conclusion that the physics of MCD at the boundary of the Brillouin zone is strongly affected by the strength of two relativistic effects in particular compounds: (i) the mass-velocity term that controls the distance of the conduction band at the L point to the upper Hubbard band of Mn ions and, thus, a relative magnitude and sign of the exchange splittings in the conduction and valence bands; (ii) the spin-momentum locking by spin-orbit coupling that reduces exchange splitting depending on the orientation of particular L valleys with respect to the magnetization direction.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic instabilities in quasi-one-dimensional Cr-based material

The magnetic response of a K2Cr3As3 sample has been studied by means of dc magnetization measurements as a function of magnetic field (H) at different temperatures ranging from 5 K up to 300 K. Looking at the magnetic hysteresis loops m(H), a diamagnetic behavior of the sample has been inferred at temperatures higher than 60 K, whereas at lower temperatures the sample shows a hysteresis loop compatible with the presence of ferrimagnetism. Moreover, several spike-like magnetization jumps, both positive and negative, have been observed at certain fields in the range -1000 Oe < H < 1000 Oe, regardless of the temperature considered. The field position of the magnetization jumps has been studied at different temperatures, and their distribution can be described by a Lorentzian curve.

cond-mat.supr-con↗

arXiv2021

Spatially modulated, orbital selective ferromagnetism in La$_5$Co$_2$Ge$_3$

We present density functional theory calculations for low-$T_c$ metallic ferromagnet La$_5$Co$_2$Ge$_3$ at ambient and applied pressures. Our investigations reveal that the system is a quasi-one-dimensional ferromagnet with a peculiar coexistence of two different orbital-selective magnetic moments at two crystallographically inequivalent cobalt atoms, Co1 and Co2. Namely, due to different crystal-field splitting, the magnetic moment of Co1 atoms predominantly derives from $d_{xz}$ orbital whereas of Co2 atoms from $d_{xy}$ orbital. Consequently, Co1 and Co2 atoms develop unequal net magnetic moments, a feature that gives rise to a periodic, spatial modulation of magnetization along crystallographic $c$-direction. The amplitude of the spatial modulation, small at ambient pressure, drastically increases with applied pressure, until Co2 atoms become nonmagnetic. With the help of a toy model mimicking found orbital-selective ferromagnetic order, we demonstrate that the increasing amplitude of spatial modulation provides a consistent interpretation to the recently observed resistivity anomaly emerging at applied pressure identified as the appearance of the {\it new state}. Although, proposed here structural origin of the spatial modulation of magnetic moments in La$_5$Co$_2$Ge$_3$ is an alternative one to the advocated for this material ferromagnetic quantum criticality avoidance, the effects of quantum fluctuations can still play an important role at pressure larger than up-to-date measured 5GPa.

cond-mat.str-el↗

arXiv2021

Tuning interchain ferromagnetic instability in A2Cr3As3 ternary arsenides by chemical pressure and uniaxial strain

We analyze the effects of chemical pressure induced by alkali metal substitution and uniaxial strain on magnetism in the A2Cr3As3 (A = Na, K, Rb, Cs) family of ternary arsenides with quasi-one dimensional structure. Within the framework of the density functional theory, we predict that the non-magnetic phase is very close to a 3D collinear ferrimagnetic state, which realizes in the regime of moderate correlations, such tendency being common to all the members of the family with very small variations due to the different interchain ferromagnetic coupling. We uncover that the stability of such interchain ferromagnetic coupling has a non-monotonic behavior with increasing the cation size, being critically related to the degree of structural distortions which is parametrized by the Cr-As-Cr bonding angles along the chain direction. In particular, we demonstrate that it is boosted in the case of the Rb, in agreement with recent experiments. We also show that uniaxial strain is a viable tool to tune the non-magnetic phase towards an interchain ferromagnetic instability. The modifcation of the shape of the Cr triangles within the unit cell favors the formation of a net magnetization within the chain and of a ferromagnetic coupling among the chains. This study can provide relevant insights about the interplay between superconductivity and magnetism in this class of materials.

cond-mat.str-el↗

arXiv2021

Intra-chain collinear magnetism and inter-chain magnetic phases in Cr3As3-K-based materials

We perform a comparative study of the KCr3As3 and the K2Cr3As3 quasi 1D compounds, and show that the strong interplay between the lattice and the spin degrees of freedom promotes a new collinear ferrimagnetic ground state within the chains in presence of intrachain antiferromagnetic couplings. We propose that the interchain antiferromagnetic coupling in KCr3As3 plays a crucial role for the experimentally observed spin-glass phase with low critical temperature. In the same region of the parameter space, we predict K2Cr3As3 to be non-magnetic but on the verge of the magnetism, sustaining interchain ferromagnetic spin fluctuations while the intrachain spin fluctuations are antiferromagnetic.

cond-mat.str-el↗

arXiv2019

Spin-orbit coupling effects on the electronic properties of the pressure-induced superconductor CrAs

We present the effects of spin-orbit coupling on the low-energy bands and Fermi surface of the recently discovered pressure-induced superconductor CrAs. We apply the Löwdin down-folding procedure to a tight-binding hamiltonian that includes the intrinsic spin-orbit interaction, originating from the Cr 3d electrons as well as from As 4p ones. Our results indicate that As contributions have negligible effects, whereas the modifications to the band structure and the Fermi surface can be mainly ascribed to the Cr contribution. We show that the inclusion of the spin-orbit interaction allows for a selective removal of the band degeneracy due to the crystal symmetries, along specific high symmetry lines. Such release of the band degeneracy naturally determines a reconstruction of the Fermi surface, including the possibility of changing the number of pockets.

cond-mat.supr-con↗

arXiv2019

Multiple band-crossings and Fermi surface topology: role of double nonsymmorphic symmetries in MnP-type crystal structures

We use relativistic ab-initio methods combined with model Hamiltonian approaches to analyze the normal-phase electronic and structural properties of the recently discovered WP superconductor. Remarkably, the outcomes of such study can be employed to set fundamental connections among WP and the CrAs and MnP superconductors belonging to the same space group. One of the key features of the resulting electronic structure is represented by the occurrence of multiple band crossings along specific high symmetry lines of the Brilloiun zone. In particular, we demonstrate that the eight-fold band degeneracy obtained along the SR path at (kx,ky)=(Pi,Pi) is due to inversion-time reversal invariance and a pair of nonsymmorphic symmetries. The presence of multiple degenerate Fermi points along the SR direction constraints the topology of the Fermi surface, which manifests distinctive marks when considering its evolution upon band filling variation. If the Fermi level crosses the bands along the SR line as it happens at the nominal filling of the MnP, these Fermi surfaces are open or closed Fermi pockets. Moving the relative position of the Fermi level away from the eight-fold degenerate bands as for the WP and CrAs compounds, the electronic changeover exhibits a simultaneous modification of the Fermi surface dimensionality and topology. Four two-dimensional (2D) Fermi surface sheets are centered around the SR line with a corrugated profile along the kz direction. Moreover, we show that the spin-orbit interaction determines a selective removal of the band degeneracy and, consequently, a splitting of the quasi 2D Fermi sheets, as it happens in WP. Finally, we comment on the connections between our results and recent experimental and theoretical proposals about the triplet superconductivity in this class of compounds.

cond-mat.supr-con↗

arXiv2019

A minimal tight-binding model for the quasi-one-dimensional superconductor K2Cr3As3

We present a systematic derivation of a minimal five-band tight-binding model for the description of the electronic structure of the recently discovered quasi one-dimensional superconductor K2Cr3As3. Taking as a reference the density-functional theory (DFT) calculation, we use the outcome of a Lowdin procedure to refine a Wannier projection and fully exploit the predominant weight at the Fermi level of the states having the same symmetry of the crystal structure. Such states are described in terms of five atomic-like d orbitals: four planar orbitals, two dxy and two dx2-y2, and a single out-of-plane one, dz2 . We show that this minimal model reproduces with great accuracy the DFT band structure in a broad energy window around the Fermi energy. Moreover, we derive an explicit simplified analytical expression of such model, which includes three nearest-neighbor hopping terms along the z direction and one nearest-neighbor term within the xy plane. This model captures very efficiently the energy spectrum of the system and, consequently, can be used to study transport properties, superconductivity and dynamical effects in this novel class of superconductors.

cond-mat.supr-con↗

arXiv2018

Optimizing the tight-binding parametrization of the quasi-one-dimensional superconductor K2Cr3As3

We study the tight-binding dispersion of the recently discovered superconductor K2Cr3As3, obtained from Wannier projection of Density Functional Theory (DFT) results. In order to establish quantitatively the actual degree of quasi-one dimensionality of this compound, we analyze the electronic band structure for two reduced sets of hopping parameters: one restricted to the Cr-As tubes and another one retaining a minimal number of in-plane hoppings. The corresponding total and local density of states of the compound are also computed with the aim of assessing the tight-binding results with respect to the DFT ones. We find a quite good agreement with the DFT results for the more extended set of hopping parameters, especially for what concerns the orbitals that dominate at the Fermi level. Accordingly, we conclude that one cannot avoid taking into account in-plane hoppings up to the next-nearest-neighbors cells even only to describe correctly the Fermi surface cuts and the populations along the kz direction. Such a choice of a minimal number of hopping parameters directly reflects in the possibility of correctly describing correlations and magnetic interactions.

cond-mat.supr-con↗

arXiv2017

Low energy bands and transport properties of chromium arsenide

We apply a method that combines the tight-binding approximation and the Lowdin down-folding procedure to evaluate the electronic band structure of the newly discovered pressure-induced superconductor CrAs. By integrating out all low-lying arsenic degrees of freedom, we derive an effective Hamiltonian model describing the Cr d bands near the Fermi level. We calculate and make predictions for the energy spectra, the Fermi surface, the density of states and transport and magnetic properties of this compound. Our results are consistent with local-density approximation calculations as well as they show good agreement with available experimental data for resistivity and Cr magnetic moment.

cond-mat.supr-con↗