arXiv subjects

Giorgio Sangiovanni

Publications and source records attributed to Giorgio Sangiovanni.

At least 109 records · Page 6Linked to original sources

arXiv2017

DFT+DMFT study on soft moment magnetism and covalent bonding in SrRu$_2$O$_6$

Dynamical mean-field theory is used to study the three orbital model of the $d^3$ compound SrRu$_2$O$_6$ both with and without explicitly including the O-$p$ states. Depending on the size of the Hund's coupling $J$, at low to intermediate temperatures we find solutions corresponding to Mott or correlated covalent insulator. The latter can explain the experimentally observed absence of Curie susceptibility in the paramagnetic phase. At high temperatures a single phase with smoothly varying properties is observed. SrRu$_2$O$_6$ provides an ideal system to study the competition between the local moment physics and covalent bonding since both effects are maximized, while spin-orbit interaction is found to play a minor role.

cond-mat.str-el↗

arXiv2017

Strongly correlated double Dirac fermions

Double Dirac fermions have recently been identified as possible quasiparticles hosted by three-dimensional crystals with particular non-symmorphic point group symmetries. Applying a combined approach of ab-initio methods and dynamical mean field theory, we investigate how interactions and double Dirac band topology conspire to form the electronic quantum state of Bi$_2$CuO$_4$. We derive a downfolded eight-band model of the pristine material at low energies around the Fermi level. By tuning the model parameters from the free band structure to the realistic strongly correlated regime, we find a persistence of the double Dirac dispersion until its constituting time reveral symmetry is broken due to the onset of magnetic ordering at the Mott transition. We analyze pressure as a promising route to realize a double-Dirac metal in Bi$_2$CuO$_4$.

cond-mat.str-el↗

arXiv2016

Robust spin-polarized midgap states at step edges of topological crystalline insulators

Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three- dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight- binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This non-trivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.

cond-mat.mes-hall↗

arXiv2016

Chromium analogues of Iron-based superconductors

We theoretically investigate the $d^4$ (Cr$^{2+}$) compound BaCr$_2$As$_2$ and show that, despite non-negligible differences in the electronic structure, its many-body physics mirrors that of BaFe$_2$As$_2$, which has instead a $d^6$ (Fe$^{2+}$) configuration. This reflects a symmetry of the electron correlation effects around the half-filled $d^5$ Mott insulating state. The experimentally known metallic antiferromagnetic phase is correctly modeled by dynamical mean-field theory, and for realistic values of the interaction it shows a moderate mass enhancement of order $\sim$2. This value decreases if the ordered moment grows as a result of a stronger interaction. The antiferromagnetic phase diagram for this $d^4$ shows similarities with that calculated for the $d^6$ systems. Correspondingly, in the paramagnetic phase the influence of the half-filled Mott insulator shows up as a crossover from a weakly correlated to an orbitally differentiated "Hund's metal" phase which reflects an analogous phenomenon in $d^6$ iron compounds including a strong enhancement of the compressibility in a zone just inside the frontier between the normal and the Hund's metal. The experimental evidence and our theoretical description place BaCr$_2$As$_2$ at interaction strength slightly below the crossover which implies that negative pressures and/or electron doping (e.g. Cr $\rightarrow$ Mn,Fe or Ba $\rightarrow$ Sc,Y,La) might strongly enhance the compressibility, thereby possibly inducing a pairing instability in this non-superconducting compound.

cond-mat.str-el↗

arXiv2016

Worm Improved Estimators in Continuous-time Quantum Monte Carlo

We derive the improved estimators for general interactions and employ these for the continuous-time quantum Monte Carlo method. Using a worm algorithm we show how measuring higher-ordered correlators leads to an improved high-frequency behavior in irreducible quantities such as the one-particle self-energy or the irreducible two-particle vertex for non-density-density interactions. A good knowledge of the asymptotics of the two-particle vertex is essential for calculating non-local electronic correlations using diagrammatic extensions to the dynamical mean field theory as well as for calculating susceptibilities. We test our algorithm against analytic results for the multi-orbital atomic-limit and the Falicov-Kimball model.

cond-mat.str-el↗

arXiv2016

Screened moments and absence of ferromagnetism in FeAl

While the stoichiometric intermetallic compound FeAl is found to be paramagnetic in experiment, standard band-theory approaches predict the material to be ferromagnetic. We show that this discrepancy can be overcome by a better treatment of electronic correlations with density functional plus dynamical mean field theory. Our results show no ferromagnetism down to 100 K and since the susceptibility is decreasing at the lowest temperatures studied we also do not expect ferromagnetism at even lower temperatures. This behavior is found to originate from temporal quantum fluctuations that screen short-lived local magnetic moments of 1.6 $μ_B$ on Fe.

cond-mat.str-el↗

arXiv2015

Continuous-time Quantum Monte Carlo using Worm Sampling

We present a worm sampling method for calculating one- and two-particle Green's functions using continuous-time quantum Monte Carlo simulations in the hybridization expansion (CT-HYB). Instead of measuring Green's functions by removing hybridization lines from partition function configurations, as in conventional CT-HYB, the worm algorithm directly samples the Green's function. We show that worm sampling is necessary to obtain general two-particle Green's functions which are not of density-density type and that it improves the sampling efficiency when approaching the atomic limit. Such two-particle Green's functions are needed to compute off-diagonal elements of susceptibilities and occur in diagrammatic extensions of the dynamical mean field theory and efficient estimators for the single-particle self-energy.

cond-mat.str-el↗

arXiv2015

Interacting weak topological insulators and their transition to Dirac semimetal phases

Topological insulators in the presence of strong Coulomb interaction constitute novel phases of matter. Transitions between these phases can be driven by single-particle or many-body effects. On the basis of {\it ab-initio} calculations, we identify a concrete material, {\it i.e.} Ca$_{2}$PtO$_{4}$, that turns out to be a hole-doped weak topological insulator. Interestingly, the Pt-$d$ orbitals in this material are relevant for the band inversion that gives rise to the topological phase. Therefore, Coulomb interaction should be of importance in Ca$_{2}$PtO$_{4}$. To study the influence of interactions on the weak topological insulating phase, we look at a toy model corresponding to a layer-stacked 3D version of the Bernevig-Hughes-Zhang model with local interactions. For small to intermediate interaction strength, we discover novel interaction-driven topological phase transitions between the weak topological insulator and two Dirac semimetal phases. The latter correspond to gapless topological phases. For strong interactions, the system eventually becomes a Mott insulator.

cond-mat.str-el↗

arXiv2015

Nickel-Titanium double perovskite: A three-dimensional spin-1 Heisenberg antiferromagnet

The double perovskite ${\rm La}_2{\rm NiTiO}_6$ is identified as a three-dimensional $S=1$ quantum magnet. By means of Density Functional Theory we demonstrate that this material is a high-spin $d$-electron system deep in the Heisenberg limit and establish that its paramagnetic Mott phase persists down to low temperatures ($T_{\rm N}$=25K) not because of frustration effects but rather for the extreme strong coupling physics. Our many-body calculations on an $ab$ $initio$-derived multi-orbital basis predict indeed a kinetic energy gain when entering the magnetically ordered phase. ${\rm La}_2{\rm NiTiO}_6$ emerges thus as a paradigmatic realization of a spin-triplet Mott insulator. Its peculiar properties may turn out to be instrumental in the ongoing chase after correlated topological states of matter.

cond-mat.str-el↗

arXiv2013

Electronics with correlated oxides: SrVO$_3$/SrTiO$_3$ as a Mott transistor

We employ density functional theory plus dynamical mean field theory and identify the physical origin of why two layers of SrVO$_3$ on a SrTiO$_3$ substrate are insulating: the thin film geometry lifts the orbital degeneracy which in turn triggers a Mott-Hubbard transition. Two layers of SrVO$_3$ are just at the verge of a Mott-Hubbard transition and hence ideally suited for technological applications of the Mott-Hubbard transition: the heterostructure is highly sensitive to strain, electric field, and temperature. A gate voltage can also turn the insulator into a metal, so that a transistor with ideal on-off (metal-insulator) switching properties is realized.

cond-mat.str-el↗

arXiv2013

Importance of d-p Coulomb interaction for high T$_C$ cuprates and other oxides

Current theoretical studies of electronic correlations in transition metal oxides typically only account for the local repulsion between d-electrons even if oxygen ligand p-states are an explicit part of the effective Hamiltonian. Interatomic interactions such as Upd between d- and (ligand) p-electrons, as well as the local interaction between p-electrons, are neglected. Often, the relative d-p orbital splitting has to be adjusted "ad hoc" on the basis of the experimental evidence. By applying the merger of local density approximation and dynamical mean field theory (LDA+DMFT) to the prototypical case of the 3-band Emery dp model for the cuprates, we demonstrate that, without any "ad hoc" adjustment of the orbital splitting, the charge transfer insulating state is stabilized by the interatomic interaction Upd. Our study hence shows how to improve realistic material calculations that explicitly include the p-orbitals.

cond-mat.str-el↗

arXiv2013

Fluctuation driven topological Hund insulators

We investigate the role of electron-electron interaction in a two-band Hubbard model based on the Bernevig-Hughes-Zhang Hamiltonian exhibiting the quantum spin Hall (QSH) effect. By means of dynamical mean-field theory, we find that a system with topologically trivial non-interacting parameters can be driven into a QSH phase at finite interaction strength by virtue of local dynamical fluctuations. For very strong interaction, the system reenters a trivial insulating phase by going through a Mott transition. We obtain the phase diagram of our model by direct calculation of the bulk topological invariant of the interacting system in terms of its single particle Green's function.

cond-mat.str-el↗

arXiv2013

Oxide Heterostructures for Efficient Solar Cells

We propose an unexplored class of absorbing materials for high-efficiency solar cells: heterostructures of transition-metal oxides. In particular, LaVO_3 grown on SrTiO_3 has a direct band gap ~1.1 eV in the optimal range as well as an internal potential gradient, which can greatly help to separate the photo-generated electron-hole pairs. Furthermore, oxide heterostructures afford the flexibility to combine LaVO_3 with other materials such as LaFeO_3 in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory to demonstrate these features.

cond-mat.mtrl-sci↗

arXiv2012

Comparing $GW$+DMFT and LDA+DMFT for the testbed material SrVO$_3$

We have implemented the $GW$+dynamical mean field theory (DMFT) approach in the Vienna ab initio simulation package. Employing the interaction values obtained from the locally unscreened random phase approximation (RPA), we compare $GW$+DMFT and LDA+DMFT against each other and against experiment for SrVO$_3$. We observed a partial compensation of stronger electronic correlations due to the reduced $GW$ bandwidth and weaker correlations due to a larger screening of the RPA interaction, so that the obtained spectra are quite similar and well agree with experiment. Noteworthily, the $GW$+DMFT better reproduces the position of the lower Hubbard side band.

cond-mat.str-el↗

arXiv2012

Conserved quantities of SU(2)-invariant interactions for correlated fermions and the advantages for quantum Monte Carlo simulations

In the context of realistic calculations for strongly-correlated materials with $d$- or $f$-electrons the efficient computation of multi-orbital models is of paramount importance. Here we introduce a set of invariants for the SU(2)-symmetric Kanamori Hamiltonian which allows to massively speed up the calculation of the fermionic trace in hybridization-expansion continuous-time quantum Monte Carlo algorithms. As an application, we show that, exploiting this set of good quantum numbers, the study of the orbital-selective Mott-transition in systems with up to seven correlated orbitals becomes feasible.

cond-mat.str-el↗

arXiv2012

Microscopic understanding of the orbital splitting and its tuning at oxide interfaces

By means of a Wannier projection within the framework of density functional theory, we are able to identify the modified c-axis hopping and the energy mismatch between the cation bands as the main source of the $t_{2g}$ splitting around the $Γ$ point for oxide heterostructures, excluding previously proposed mechanisms such as Jahn-Teller distortions or electric field asymmetries. Interfacing LaAlO$_3$, LaVO$_3$, SrVO$_3$ and SrNbO$_3$ with SrTiO$_3$ we show how to tune this orbital splitting, designing heterostructures with more $d_{xy}$ electrons at the interface. Such an "orbital engineering" is the key for controlling the physical properties at the interface of oxide heterostructures.

cond-mat.str-el↗

arXiv2011

Cluster-size dependence in cellular dynamical mean-field theory

We examine the cluster-size dependence of the cellular dynamical mean-field theory (CDMFT) applied to the two-dimensional Hubbard model. Employing the continuous-time quantum Monte Carlo method as the solver for the effective cluster model, we obtain CDMFT solutions for 4-, 8-, 12-, and 16-site clusters at a low temperature. Comparing various periodization schemes, which are used to construct the infinite-lattice quantities from the cluster results, we find that the cumulant periodization yields the fastest convergence for the hole-doped Mott insulator where the most severe size dependence is expected. We also find that the convergence is much faster around (0,0) and (pi/2,pi/2) than around (pi,0) and (pi,pi). The cumulant-periodized self-energy seems to be close to its thermodynamic limit already for a 16-site cluster in the range of parameters studied. The 4-site results remarkably agree well with the 16-site results, indicating that the previous studies based on the 4-site cluster capture the essence of the physics of doped Mott insulators.

cond-mat.str-el↗

arXiv2011

Effects of electronic correlations and disorder on the thermopower of NaxCoO2

For the thermoelectric properties of NaxCoO2, we analyze the effect of local Coulomb interaction and (disordered) potential differences for Co-sites with adjacent Na-ion or vacancy. The disorder potential alone increases the resistivity and reduces the thermopower, while the Coulomb interaction alone leads only to minor changes compared to the one-particle picture of the local density approximation. Only combined, these two terms give rise to a substantial increase of the thermopower: the number of (quasi-)electrons around the Fermi level is much more suppressed than that of the (quasi-)holes. Hence, there is a particle-hole imbalance acting in the same direction as a similar imbalance for the group velocities. Together, this interplay results in a large positive thermopower. Introducing a thermoelectric spectral density, we located the energies and momenta regions most relevant for the thermopower and changes thereof.

cond-mat.str-el↗