5 research outputs found
Quantizzazione canonica del campo gravitazionale
Si mostra il tentativo di costruzione di una teoria quantistica del campo gravitazionale attraverso il metodo di Dirac come originariamente presentato da DeWitt nel 1967. A tal fine, si discutono i sistemi hamiltoniani vincolati e in particolare quelli generalmente covarianti, per le analogie formali che questi presentano con la Relatività Generale. Il programma di quantizzazione è presentato dopo aver introdotto il formalismo geometrico che permette di scrivere l'azione ADM del campo gravitazionale, la quale costituisce il punto di partenza per lo sviluppo della teoria quantistica. Enfasi particolare è posta sul problema del tempo e sul ruolo della covarianza generale
The Mott transition in the 5d compound BaNaOsO a DFT+DMFT study with PAW non-collinear projectors
Spin-orbit coupling has been reported to be responsible for the insulating
nature of the 5d osmate double perovskite BaNaOsO (BNOO). However,
whether spin-orbit coupling indeed drives the metal-to-insulator transition
(MIT) in this compound is an open question. In this work we investigate the
impact of relativistic effects on the electronic properties of BNOO via density
functional theory plus dynamical mean-field theory calculations in the
paramagnetic regime, where the insulating phase is experimentally observed. The
correlated subspace is modeled with non-collinear projectors of the projector
augumented wave method (PAW) employed in the Vienna Ab Initio Simulation
Package (VASP), suitably interfaced with the TRIQS package. The inclusion of
PAW non-collinear projectors in TRIQS enables the treatment of spin-orbit
coupling effects fully ab-initio within the dynamical mean-field theory
framework. In the present work, we show that spin-orbit coupling, although
assisting the MIT in BNOO, is not the main driving force for its gapped
spectra, placing this material in the Mott insulator regime. Relativistic
effects primarily impact the correlated states' character, excitations, and
magnetic ground-state properties
Spin-orbital Jahn-Teller bipolarons
Polarons and spin-orbit (SO) coupling are distinct quantum effects that play
a critical role in charge transport and spin-orbitronics. Polarons originate
from strong electron-phonon interaction and are ubiquitous in polarizable
materials featuring electron localization, in particular
transition metal oxides (TMOs). On the other hand, the relativistic coupling
between the spin and orbital angular momentum is notable in lattices with heavy
atoms and develops in TMOs, where electrons are spatially
delocalized. Here we combine ab initio calculations and magnetic measurements
to show that these two seemingly mutually exclusive interactions are entangled
in the electron-doped SO-coupled Mott insulator
(), unveiling the formation of
spin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller
lattice activity, converts the Os spin-orbital
levels, characteristic of the parent compound
(BNOO), into a bipolaron
manifold, leading to the coexistence of different
J-effective states in a single-phase material. The gradual increase of
bipolarons with increasing doping creates robust in-gap states that prevents
the transition to a metal phase even at ultrahigh doping, thus preserving the
Mott gap across the entire doping range from BNOO to
(BCOO)
Small polarons in spin-orbit coupled osmates
Small polarons (SP) have been thoroughly investigated in 3d transition metal oxides
and they have been found to play a crucial role in physical phenomena such as charge
transport, colossal magnetoresistance and surface reactivity. However, our knowledge
about these quasi-particles in 5d systems remains very limited, since the more delocalised
nature of the 5d orbitals reduces the strength of the Electronic Correlation (EC), making
SP formation in these compounds rather unexpected. Nevertheless, the Spin-Orbit coupled Dirac-Mott insulator Ba2NaOsO6 (BNOO) represents a good candidate for enabling
polaron formation in a relativistic background, due to the relatively large EC (U ∼ 3 eV)
and Jahn-Teller activity. Moreover, anomalous peaks in Nuclear Magnetic Resonance
(NMR) spectroscopy experiments suggest the presence of thermally activated SP dynamics when BNOO is doped with Ca atoms. We investigate SP formation in BNOO both
from an electronic and structural point of view by means of fully relativistic first principles calculations. Our numerical simulations predict a stable SP ground state and agree
on the value of 810 K for the dynamical process peak found by NMR experiments