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Peter E. Blöchl

Publications and source records attributed to Peter E. Blöchl.

At least 19 recordsLinked to original sources
arXiv2026

Non-magnetic insulating phase induced by Jahn-Teller effect in RNiO$_3$

We propose a three-dimensional multi-orbital tight-binding model for rare-earth nickelates RNiO$_3$ that treats charge, spin, orbital, and lattice degrees of freedom on equal footing. All model parameters, including the on-site interactions $U$ and $J$ and the electron-phonon (el-ph) coupling to the breathing mode, are extracted from hybrid-functional DFT calculations for the small-bandwidth nickelate LuNiO$_3$. The model describes three competing insulating phases governed by the interplay of $U{-}3J$ and el-ph coupling to the breathing and Jahn--Teller (JT) modes. For large $U{-}3J$, the insulating state is stabilized by local JT distortions on high-spin Ni$^{3+}$ sites. For smaller $U{-}3J$, the system undergoes charge disproportionation, $2\mathrm{Ni}^{3+}\rightarrow\mathrm{Ni}^{2+}+\mathrm{Ni}^{4+}$, resulting in the spin-polarized charge-ordered state observed experimentally below the Néel temperature in small-bandwidth RNiO$_3$. When the JT energy on the Ni$^{2+}$ site exceeds Hund's exchange $3J$, a distinct charge- and orbital-ordered insulating phase emerges in which the two $e_g$-electrons occupy the same orbital with opposite spin. The stability of this phase is further confirmed by self-consistent calculations within the full three-dimensional tight-binding model. This newly predicted metastable state, characterized by JT distortions in a nonmagnetic charge-ordered RNiO$_3$ phase, shows that the onset of magnetic order is not required for the metal-insulator transition in RNiO$_3$.

cond-mat.str-el↗
arXiv2026

Photoinduced orbital polarization and Jahn-Teller effect in RNiO$_3$

The orbital degree of freedom in rare-earth nickelates is typically inactive across the temperature-driven metal-insulator transition, where the system develops two inequivalent Ni sites associated with Ni-O bond disproportionation and breathing-mode distortions of NiO$_6$ octahedra. Here, we show that orbital polarization can be induced by optical excitation with linearly polarized light. Using an interacting multiband tight-binding model combined with real-time simulations of coupled electron-ion-spin dynamics, we find that photoinduced $d$-$d$ transitions reduce the local magnetic moments at Ni sites and effectively suppress Hund's coupling $J$ in the excited state. Importantly, these transitions can be made strongly orbital-selective by tuning the light polarization, leading to an imbalance in $e_g$ orbital occupancies. The resulting nonequilibrium state, characterized by reduced effective $J$ and unequal orbital populations, becomes unstable toward Jahn-Teller (JT) distortions, driving structural relaxation along coherently excited JT modes. Our results demonstrate that polarization-controlled optical excitation provides a pathway to access hidden nonthermal phases with emergent orbital order, enabling coherent control of coupled charge, spin, and lattice degrees of freedom on ultrafast timescales.

cond-mat.str-el↗
arXiv2026

Optical control of spin-splitting in an altermagnet

Manipulating and controlling the band structure and the spin-splitting in the newly discovered class of magnetic materials known as 'altermagnets' is highly desirable for their application in spintronics. Based on real-time simulations for an interacting multiband tight-binding model, we propose optical excitations as an effective way to selectively control the spin-splitting of an altermagnet. The consistent treatment of electronic interactions and electron-phonon coupling in the model allows for a systematic study of the effect of these interactions on the spin-splitting of the altermagnet in the ground as well as in the excited-state. Our simulations reveal that optical excitations modify the band structure and thus lead to significant changes in the spin-splitting within 50 fs. The relative spin-splitting in the conduction band grows up to four times in the optically excited altermagnet. We disentangle the roles of Coulomb $U$ and $J$ in the enhancement of the spin-splitting in the photoexcited state. Our study elucidates the potential for exploiting optical control of spin-splitting gaps to obtain desirable properties in altermagnets on the fastest possible timescales.

cond-mat.str-el↗
arXiv2026

The CP-PAW code package for first-principles calculations from a user's perspective

CP-PAW is a combined electronic structure and ab-initio molecular dynamics code to perform mixed quantum and classical simulations of atomistic condensed phase systems, such as solids, liquids, and molecular systems. As the name suggests, the CP-PAW code unifies the all-electron projector augmented-wave method with the Car-Parrinello approach to determine not only the electronic and nuclear ground state of condensed matter, but also to study their properties and dynamics. In addition to briefly outlining the underlying theory, the focus will be on unique aspects of CP-PAW and how to correctly employ them as a user. How to install CP-PAW using the new build system will also be briefly mentioned.

cond-mat.mtrl-sci↗
arXiv2022

Real-time non-adiabatic dynamics in the one-dimensional Holstein model: Trajectory-based vs exact methods

We benchmark a set of quantum-chemistry methods, including multitrajectory Ehrenfest, fewest-switches surface-hopping, and multiconfigurational-Ehrenfest dynamics, against exact quantum-many-body techniques by studying real-time dynamics in the Holstein model. This is a paradigmatic model in condensed matter theory incorporating a local coupling of electrons to Einstein phonons. For the two-site and three-site Holstein model, we discuss the exact and quantum-chemistry methods in terms of the Born-Huang formalism, covering different initial states, which either start on a single Born-Oppenheimer surface, or with the electron localized to a single site. For extended systems with up to 51 sites, we address both the physics of single Holstein polarons and the dynamics of charge-density waves at finite electron densities. For these extended systems, we compare the quantum-chemistry methods to exact dynamics obtained from time-dependent density matrix renormalization group calculations with local basis optimization (DMRG-LBO). We observe that the multitrajectory Ehrenfest method, in general, only captures the ultrashort time dynamics accurately. In contrast, the surface-hopping method with suitable corrections provides a much better description of the long-time behavior but struggles with the short-time description of coherences between different Born-Oppenheimer states. We show that the multiconfigurational Ehrenfest method yields a significant improvement over the multitrajectory Ehrenfest method and can be converged to the exact results in small systems with moderate computational efforts. We further observe that for extended systems, this convergence is slower with respect to the number of configurations. Our benchmark study demonstrates that DMRG-LBO is a useful tool for assessing the quality of the quantum-chemistry methods.

cond-mat.mtrl-sci↗
arXiv2020

Predicting Oxidation and Spin States by High-Dimensional Neural Networks: Applications to Lithium Manganese Oxide Spinels

Lithium ion batteries often contain transition metal oxides like Li$_{x}$Mn$_2$O$_4$ ($0\leq x\leq2$). Depending on the Li content different ratios of Mn$^\text{III}$ to Mn$^\text{IV}$ ions are present. In combination with electron hopping the Jahn-Teller distortions of the Mn$^\text{III}$O$_6$ octahedra can give rise to complex phenomena like structural transitions and conductance. While for small model systems oxidation and spin states can be determined using density functional theory (DFT), the investigation of dynamical phenomena by DFT is too demanding. Previously, we have shown that a high-dimensional neural network potential can extend molecular dynamics (MD) simulations of Li$_{x}$Mn$_2$O$_4$ to nanosecond time scales, but these simulations did not provide information about the electronic structure. Here we extend the use of neural networks to the prediction of atomic oxidation and spin states. The resulting high-dimensional neural network is able to predict the spins of the Mn ions with an error of only 0.03 $\hbar$. We find that the Mn e$_\text{g}$ electrons are correctly conserved and that the number of Jahn-Teller distorted Mn$^\text{III}$O$_6$ octahedra is predicted precisely for different Li loadings. A charge ordering transition is observed between 280 and 300 K, which matches resistivity measurements. Moreover, the activation energy of the electron hopping conduction above the phase transition is predicted to be 0.18 eV deviating only 0.02 eV from experiment. This work demonstrates that machine learning is able to provide an accurate representation of both, the geometric and the electronic structure dynamics of Li$_x$Mn$_2$O$_4$, on time and length scales that are not accessible by ab initio MD.

cond-mat.mtrl-sci↗
arXiv2020

Closing the gap between theory and experiment for lithium manganese oxide spinels using a high-dimensional neural network potential

Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing X-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment.

cond-mat.mtrl-sci↗
arXiv2020

A criticial view on e$_g$ occupancy as a descriptor for oxygen evolution catalytic activity in LiMn$_2$O$_4$ nanoparticles

We investigate the effect of the surface electronic structure and composition of LiMn$_2$O$_4$ nanoparticles on the electrocatalytic oxygen evolution reaction (OER). Scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) studies combined with density functional theory (DFT) based simulations of the EEL spectra reveal in pristine nanoparticles a 4 nm thick surface layer with reduced average Mn oxidation state and increased Mn concentration. This is attributed to Mn$^{2+}$ partially replacing Li$^+$ at the tetrahedral sites of the spinel lattice accompanied by Mn 3d-state filling of octahedrally coordinated Mn. During electrocatalytic OER cycling, this near-surface tetrahedral Mn is leached out, thereby increasing the oxidation state of octahedrally coordinated Mn. Using rotating ring-disc electrode (RRDE) based detection of O and Mn during the OER, we show that the oxygen evolution remains constant while the Mn$^{2+}$ is removed, revealing that near-surface tetrahedrally coordinated Mn has no effect on the OER activity of LiMn$_2$O$_4$. This is surprising since the e$_g$ occupancy of Mn in octahedral sites, which is widely used as a descriptor of OER activity, changes significantly in the surface layer during cycling. The fact that e$_g$ emptying fails to correlate with OER activity here indicates that octahedral cation valence is not a fundamental measure of activity, either because the active surface state is not affected by tetrahedral Mn or because other details of the band structure or metal-oxygen bonding character, more strongly regulate the rate-limiting steps for OER.

cond-mat.mtrl-sci↗
arXiv2020

A Hybrid Density Functional Theory Benchmark Study on Lithium Manganese Oxides

The lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$, with $0\leq x\leq 2$, is an important example for cathode materials in lithium ion batteries. However, an accurate description of Li$_x$Mn$_2$O$_4$ by first-principles methods like density functional theory is far from trivial due to its complex electronic structure, with a variety of energetically close electronic and magnetic states. It was found that the local density approximation as well as the generalized gradient approximation (GGA) are unable to describe Li$_x$Mn$_2$O$_4$ correctly. Here, we report an extensive benchmark for different Li$_x$Mn$_y$O$_z$ systems using the hybrid functionals PBE0 and HSE06, as well as the recently introduced local hybrid functional PBE0r. We find that all of these functionals yield energetic, structural, electronic, and magnetic properties in good agreement with experimental data. The notable benefit of the PBE0r functional, which relies on on-site Hartree-Fock exchange only, is a much reduced computational effort that is comparable to GGA functionals. Furthermore, the Hartree-Fock mixing factors in PBE0r are smaller than in PBE0, which improves the results for (lithium) manganese oxides. The investigation of Li$_x$Mn$_2$O$_4$ shows that two Mn oxidation states, +III and +IV, coexist. The Mn$^\text{III}$ ions are in the high-spin state and the corresponding MnO$_6$ octahedra are Jahn-Teller distorted. The ratio between Mn$^\text{III}$ and Mn$^\text{IV}$ and thus the electronic structure changes with the Li content while no major structural changes occur in the range from $x=0$ to $1$. This work demonstrates that the PBE0r functional provides an equally accurate and efficient description of the investigated Li$_x$Mn$_y$O$_z$ systems.

cond-mat.mtrl-sci↗
arXiv2020

Evolution of the magnetic and polaronic order of $\rm{Pr_{1/2}Ca_{1/2}MnO_3}$ following an ultrashort light pulse

The dynamics of electrons, spins and phonons induced by optical femtosecond pulses has been simulated for the polaronic crystal $\rm{Pr_{1/2}Ca_{1/2}MnO_3}$. The model used for the simulation has been derived from first-principles calculations. The simulations reproduce the experimentally observed melting of charge/orbital order with increasing fluence. The loss of charge order in the high-fluence regime induces a transition to a ferromagnetic metal. At low fluence, the dynamics is deterministic and coherent phonons are created by the repopulation of electronic orbitals, which are strongly coupled to the phonon degrees of freedom. In contrast to the low-fluence regime, the magnetic transitions occurring at higher fluence can be attributed to a quasi-thermal transition of a cold-plasma-like state with hot electrons and cold phonons and spins. The findings can be rationalized in a more complete picture of the electronic structure that goes beyond the simple ionic picture of charge order.

cond-mat.str-el↗
arXiv2018

Relaxation of photoexcitations in polaron-induced magnetic microstructures

We investigate the evolution of a photoexcitation in correlated materials over a wide range of time scales. The system studied is a one-dimensional model of a manganite with correlated electron, spin, orbital, and lattice degrees of freedom, which we relate to the three-dimensional material Pr$_{1-x}$Ca$_{x}$MnO$_3$. The ground-state phases for the entire composition range are determined and rationalized by a coarse-grained polaron model. At half-doping a pattern of antiferromagnetically coupled Zener polarons is realized. Using time-dependent density-matrix renormalization group (tDMRG), we treat the electronic quantum dynamics following the excitation. The emergence of quasiparticles is addressed, and the relaxation of the nonequilibrium quasiparticle distribution is investigated via a linearized quantum-Boltzmann equation. Our approach shows that the magnetic microstructure caused by the Zener polarons leads to an increase of the relaxation times of the excitation.

cond-mat.str-el↗
arXiv2018

Adaptive cluster approximation for reduced density-matrix functional theory

A method, called the adaptive cluster approximation (ACA), for single-impurity Anderson models is proposed. It is based on reduced density-matrix functional theory, where the one-particle reduced density matrix is used as the basic variable. The adaptive cluster approximation introduces a unitary transformation of the bath states such that the effect of the bath is concentrated to a small cluster around the impurity. For this small effective system one can then either calculate the reduced density-matrix functional numerically exact from Levy's constrained-search formalism or approximate it by an implicit approximation of the reduced density-matrix functional. The method is evaluated for single-impurity Anderson models with finite baths. The method converges rapidly to the exact result with the size of the effective bath.

cond-mat.str-el↗
arXiv2016

Reduced density-matrix functionals applied to the Hubbard dimer

Common density-matrix functionals, the Müller and the power functional, have been benchmarked for the half-filled Hubbard dimer, which allows to model the bond dissociation problem and the transition from the weakly to the strongly correlated limit. Unbiased numerical calculations are combined with analytical results. Despite the well known successes of the Müller functional, the ground state is degenerate with a one-dimensional manifold of ferromagnetic solutions. The resulting infinite magnetic susceptibility indicates another qualitative flaw of Müller's functional. The derivative discontinuity with respect to particle number is not present indicating an incorrect metal-like behavior. The power functional actually favors the ferromagnetic state for weak interaction. Analogous to the Hartree-Fock approximation, the power functional undergoes a transition beyond a critical interaction strength, in this case however, to a non-collinear antiferromagnetic state.

cond-mat.str-el↗
arXiv2015

Electronic structure and spin polarization of Fe$_{1-x}$Mn(Co,Ni)$_x$S$_2$ alloys from first principles

Alloying effects by T=Mn,Co,Ni-substitution on FeS$_2$ have been investigated using density-functional calculations. The ferromagnetic alloys Fe$_{1-x}$T$_x$S$_2$ have been investigated for concentrations $x=\frac{1}{4},\frac{1}{2},\frac{3}{4}$ together with the ground states of the pure compounds. The electronic structure is discussed with the goal to identify candidates for half metals, which are of interest for spintronics applications. We find interesting candidates at high concentration of the Mn-doped FeS$_2$ and at low concentrations for Ni-doped materials. For the Mn alloys we also note the proximity to a low-spin to high spin transition. For Co-doped materials we reproduce the well known finding of half metallicity over the entire concentration range.

cond-mat.mtrl-sci↗
arXiv2013

Density-matrix functionals from Greens functions

The exact reduced density-matrix functional is derived from the Luttinger-Ward functional of the single-particle Green's function. Thereby, a formal link is provided between diagrammatic many-body approaches using Green's functions on the one hand and theories based on many-body wave functions on the other. This link can be used to explicitly construct approximations for the density-matrix functional that are equivalent to standard diagrammatic re-summation techniques and to non-perturbative dynamical mean-field theory in particular. Contrary to functionals of the Green's-function, the exact density-matrix functional is convex and thus provides a true minimum principle which facilitates the calculation of the grand potential and derived equilibrium properties. The benefits of the proposed Green's-function-based density-matrix functional theory for geometrical structure optimization of strongly correlated materials are discussed.

cond-mat.str-el↗
arXiv2012

Node-less atomic wave functions, Pauli repulsion and systematic projector augmentation

A construction of node-less atomic orbitals and energy-dependent, node-reduced partial waves is presented, that contains the full information of the atomic eigenstates and that allows to represent the scattering properties in a transparent manner. By inverting the defining Schrödinger equation, the Pauli repulsion by the core electrons can be represented as effective potential. This construction also provides a description of the Pauli repulsion by an environment. Furthermore, the representation leads to a new systematic scheme for a projector augmentation. The relation to Slater orbitals is discussed.

physics.chem-ph↗
arXiv2011

Theory and Practice of Density-Functional Theory

A introduction into density-functional theory and electronic structure methods is given, that aims at providing an intuitive understanding of the underlying concepts for the novice as well as an entry point towards the more advanced scientific literature.

cond-mat.other↗