Symmetry breaking in vanadium trihalides

In the light of new experimental evidence we study the insulating ground state of the $3d^2$-transition metal trihalides VX$_3$ (X=Cl, I). Based on Density Functional Theory with the Hubbard correction (DFT$+U$) we systematically show how these systems host multiple metastable states characterized by different orbital ordering and electronic behaviour. Our calculations reveal the importance of imposing a precondition in the on site $d$ density matrix and of considering a symmetry broken unit cell to correctly take into account the correlation effects in a mean field framework. Furthermore we ultimately found a ground state with the $a_{1g}$ orbital occupied in a distorted VX$_6$ octahedra driven by an optical phonon mode.Comment: 6 pages, 4 figures + supplementar

How to make graphene superconducting

Graphene is the physical realization of many fundamental concepts and phenomena in solid state-physics, but in the long list of graphene remarkable properties, a fundamental block is missing: superconductivity. Making graphene superconducting is relevant as the easy manipulation of this material by nanolytographic techniques paves the way to nanosquids, one-electron superconductor-quantum dot devices, superconducting transistors at the nano-scale and cryogenic solid-state coolers. Here we explore the doping of graphene by adatoms coverage. We show that the occurrence of superconductivity depends on the adatom in analogy with graphite intercalated compounds (GICs). However, most surprisingly, and contrary to the GIC case, Li covered graphene is superconducting at much higher temperature with respect to Ca covered graphene

Elemental Phosphorus: structural and superconducting phase diagram under pressure

Pressure-induced superconductivity and structural phase transitions in phosphorous (P) are studied by resistivity measurements under pressures up to 170 GPa and fully $ab-initio$ crystal structure and superconductivity calculations up to 350 GPa. Two distinct superconducting transition temperature (T$_{c}$) vs. pressure ($P$) trends at low pressure have been reported more than 30 years ago, and for the first time we are able to reproduce them and devise a consistent explanation founded on thermodynamically metastable phases of black-phosphorous. Our experimental and theoretical results form a single, consistent picture which not only provides a clear understanding of elemental P under pressure but also sheds light on the long-standing and unsolved $anomalous$ superconductivity trend. Moreover, at higher pressures we predict a similar scenario of multiple metastable structures which coexist beyond their thermodynamical stability range. Metastable phases of P experimentally accessible at pressures above 240 GPa should exhibit T$_{c}$'s as high as 15 K, i.e. three times larger than the predicted value for the ground-state crystal structure. We observe that all the metastable structures systematically exhibit larger transition temperatures than the ground-state ones, indicating that the exploration of metastable phases represents a promising route to design materials with improved superconducting properties.Comment: 14 pages, 4 figure

Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach

We develop a first-principles scheme to calculate adiabatic and non-adiabatic phonon frequencies in the full Brillouin zone. The method relies on the variational properties of a force-constants functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon dispersion curves free from convergence issues related to Brillouin zone sampling. In addition our approach justify the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in MgB$_2$ and CaC$_6$. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron and phonon momentum integration. In MgB$_2$, the presence of Kohn anomalies on the E$_{2g}$ branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In CaC$_6$ we show that the non-adiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens new perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure

A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials

Two hydrogen-rich materials, H$_3$S and LaH$_{10}$, synthesized at megabar pressures, have revolutionized the field of condensed matter physics providing the first glimpse to the solution of the hundred-year-old problem of room temperature superconductivity. The mechanism underlying superconductivity in these exceptional compounds is the conventional electron-phonon coupling. Here we describe recent advances in experimental techniques, superconductivity theory and first-principles computational methods which have made possible these discoveries. This work aims to provide an up-to-date compendium of the available results on superconducting hydrides and explain how the synergy of different methodologies led to extraordinary discoveries in the field. Besides, in an attempt to evidence empirical rules governing superconductivity in binary hydrides under pressure, we discuss general trends in the electronic structure and chemical bonding. The last part of the Review introduces possible strategies to optimize pressure and transition temperatures in conventional superconducting materials as well as future directions in theoretical, computational and experimental research.Comment: 68 pages, 30 figures, Preprint submitted to Physics Report

Polaronic and Mott insulating phase of layered magnetic vanadium trihalide VCl3

Two-dimensional (2D) van der Waals (vdW) magnetic $3d$-transition metal trihalides are a new class of functional materials showing exotic physical properties useful for spintronic and memory storage applications. This letter presents the synthesis and electromagnetic characterization of single-crystalline vanadium trichloride, VCl$_3$, a novel 2D layered vdW Mott insulator, which has a rhombohedral structure (R$\overline{3}$, No. 148) at room temperature. VCl$_3$ undergoes a structural phase transition at 103 K and a subsequent antiferromagnetic transition at 21.8 K. Combining core levels and valence bands X-ray Photoemission Spectroscopy (XPS) with first-principles Density Functional Theory (DFT) calculations, we demonstrate the Mott Hubbard insulating nature of VCl$_3$ and the existence of electron small 2D magnetic polarons localized on V atom sites by V-Cl bond relaxation. The polarons strongly affect the electromagnetic properties of VCl$_3$ promoting the occupation of dispersion-less spin-polarized V-3d $a_{1g}$ states and band inversion with $e^{'}_{g}$ states. Within the polaronic scenario, it is possible to interpret different experimental evidences on vanadium trihalides, such as VI$_3$, highlighting the complex physical behavior determined by correlation effects, mixed valence states, and magnetic states

Topological band inversion in HgTe(001): surface and bulk signatures from photoemission

HgTe is a versatile topological material and has enabled the realization of a variety of topological states, including two- and three-dimensional (3D) topological insulators and topological semimetals. Nevertheless, a quantitative understanding of its electronic structure remains challenging, in particular due to coupling of the Te 5p-derived valence electrons to Hg 5d core states at shallow binding energy. We present a joint experimental and theoretical study of the electronic structure in strained HgTe(001) films in the 3D topological-insulator regime, based on angle-resolved photoelectron spectroscopy and density functional theory. The results establish detailed agreement in terms of (i) electronic band dispersions and orbital symmetries, (ii) surface and bulk contributions to the electronic structure, and (iii) the importance of Hg 5d states in the valence-band formation. Supported by theory, our experiments directly image the paradigmatic band inversion in HgTe, underlying its non-trivial band topology

Preface Special Issue on novel superconducting and magnetic materials

Superconductivity and magnetism -- and their entanglement in a single material -- are among the most studied phenomena in condensed matter physics and continue to pose new challenges for fundamental research and exciting opportunities for technological applications. The last decade has witnessed ground-breaking discoveries in both fields: high-temperature superconductivity in compressed hydrides, unconventional superconductivity in iron-based materials and new types of magnetic states in spin-orbit coupled materials with topological and nematic characteristics. The prediction of material-specific properties and the interpretation of superconducting and magnetic phase transitions have been crucially aided by advances in ab-initio electronic structure methods within the density functional theory and its extensions. This special issue gathers together selected theoretical and experimental contributions on novel aspects of superconductivity and magnetism, %that have been collected in memory of Prof. Sandro Massidda. The collection aims to provide an updated view on timing issues and challenges in this active research field that have been at the hearth of Sandro's scientific interests. As commemorated in the obituary by Continenza and Colombo, Sandro has dedicated his scientific work to the development and application of \textit{ab-initio} computational and theoretical methods, yet never losing focus to the ultimate goal of theoretical and computational physics, that is to support, complement and understand the experimental observations

The 2021 room-temperature superconductivity roadmap.

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms.In memoriam, to Neil Ashcroft, who inspired us all