We present high temperature inelastic neutron scattering and magnetic susceptibility measurements of the antiferromagnetic insulator LaMnPO that are consistent with the presence of two-dimensional magnetic correlations up to a temperature $T_{\text{max}}\approx 700\text{K}\gg T_N=375$ K, the Néel temperature. Optical transmission measurements show the $T=300$ K direct charge gap $\Delta =1$ eV has decreased only marginally by 500 K and suggest it decreases by only 10% at $T_{\text{max}}$. Density functional theory and dynamical mean-field theory calculations reproduce a direct charge gap in paramagnetic LaMnPO only when a strong Hund's coupling $J_H=0.9$ eV is included, as well as on-site Hubbard $U=8$ eV. Our results show that LaMnPO is a Mott-Hund's insulator, in which the charge gap is rather insensitive to antiferromagnetic exchange coupling.

Electric polarization in conventional paraelectric/ferroelectric oxides usually involves the displacement of nonmagnetic transition-metal ions with an empty $d$ shell. Here we unravel an unusual mechanism for electric polarization based on the displacement of magnetic ${\mathrm{Fe}}^{3+}$ $\left(3d^5\right)$ ions. Our simulations suggest that the competition between the long-range Coulomb interaction and short-range Pauli repulsion in a $\mathrm{Fe}{\mathrm{O}}_5$ bipyramidal unit with proper lattice parameters would favor an off-center displacement of ${\mathrm{Fe}}^{3+}$, which directly induces a local electric dipole. As a prototype example, we show that the electric dipole of a $\mathrm{Fe}{\mathrm{O}}_5$ bipyramid in $(\mathrm{Ba},\mathrm{Sr})\mathrm{F}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ hexaferrites leads to a different family of magnetic quantum paraelectrics. The manipulation of this unique magnetic-ion-induced displacive electric polarization in a $\mathrm{Fe}{\mathrm{O}}_5$ bipyramid could open up a promising route to generating unconventional dielectrics, ferroelectrics, and multiferroics.

We construct a family of exactly solvable spin models that illustrate a mechanism for fractionalization in topologically ordered phases, dubbed the string flux mechanism. The essential idea is that an anyon of a topological phase can be endowed with fractional quantum numbers when the string attached to it slides over a background pattern of flux in the ground state. The string flux models that illustrate this mechanism are ${\mathbb{Z}}_n$ quantum double models defined on specially constructed $d$-dimensional lattices, and possess ${\mathbb{Z}}_n$ topological order for $d\ge 2$. The models have a unitary, internal symmetry $G$, where $G$ is an arbitrary finite group. The simplest string flux model is a ${\mathbb{Z}}_2$ toric code defined on a bilayer square lattice, where $G={\mathbb{Z}}_2$ is layer-exchange symmetry. In general, by varying the pattern of ${\mathbb{Z}}_n$ flux in the ground state, any desired fractionalization class [element of $H^2(G,{\mathbb{Z}}_n)]$ can be realized for the ${\mathbb{Z}}_n$ charge excitations. While the string flux models are not gauge theories, they map to ${\mathbb{Z}}_n$ gauge theories in a certain limit, where they follow a magnetic route for the emergence of low-energy gauge structure. The models are analyzed by studying the action of $G$ symmetry on ${\mathbb{Z}}_n$ charge excitations, and by gauging the $G$ symmetry. The latter analysis confirms that distinct fractionalization classes give rise to distinct quantum phases, except that classes $\left[\omega \right],{\left[\omega \right]}^{-1}\in H^2(G,{\mathbb{Z}}_n)$ give rise to the same phase. We conclude with a discussion of open issues and future directions.

Among the factors determining the quantum coherence of the spin in molecular magnets are the presence and the nature of nuclear spins in the molecule. We have explored modifying the nuclear-spin environment in ${\mathrm{Cr}}_7\mathrm{Ni}$-based molecular nanomagnets by replacing hydrogen atoms with deuterium or the halogen atoms, fluorine or chlorine. We find that the spin coherence, studied at low temperatures by pulsed electron-spin resonance, is modified by a range of factors, including nuclear spin and magnetic moment, changes in dynamics owing to nuclear mass, and molecular morphology changes.

Employing a combination of functional renormalization group calculations and projective determinantal quantum Monte Carlo simulations, we examine the Hubbard model on the square lattice bilayer at half filling. From this combined analysis, we obtain a comprehensive account of the ground-state phase diagram with respect to the extent of the system's metallic and (antiferromagnetically ordered) Mott-insulating as well as band-insulating regions. By means of an unbiased functional renormalization group approach, we exhibit the antiferromagnetic Mott-insulating state as the relevant instability of the free metallic state, induced by any weak finite on-site repulsion. Upon performing a careful analysis of the quantum Monte Carlo data, we resolve the difficulty of identifying this antiferromagnetic ground state for finite interlayer hopping in the weak-coupling regime, where nonmonotonic finite-size corrections are shown to relate to the two-sheeted Fermi surface structure of the metallic phase. On the other hand, quantum Monte Carlo simulations are well suited to identify the transition between the Mott-insulating phase and the band insulator in the intermediate-to-strong coupling regime. Here, we compare our numerical findings to indications for the transition region obtained from the functional renormalization group procedure.

We provide conclusive experimental evidence that zero-bias anomaly in the differential resistance of magnetic tunnel junctions is due to electron-electron interaction (EEI), clarifying a longstanding issue. The magnon effect that caused confusion is now excluded by measuring at low temperatures down to 0.2 K and with reduced ac measurement voltages down to 0.06 mV. The normalized change of conductance is proportional to $ln(eV/k_{B}T)$, consistent with the Altshuler-Aronov theory of tunneling that describes the reduction of density of states due to EEI, but inconsistent with magnetic impurity scattering. The slope of the $ln(eV/k_{B}T)$ dependence is symmetry dependent: the slopes for parallel and antiparallel states are different for coherent tunnel junctions with symmetry filtering, while nearly the same for those without symmetry filtering (amorphous barriers). This observation may be helpful for verifying symmetry-preserved filtering in search of new coherent tunneling junctions, and for probing and separating electron Bloch states of different symmetries in other correlated systems.

We study the half-filled Hubbard model on a one-parameter family of vortex-full square lattices ranging from the isotropic case to weakly coupled Hubbard dimers. The ground-state phase diagram consists of four phases: A semimetal and a band insulator which are connected to the weak-coupling limit, and a magnetically ordered Néel phase and a valence bond solid (VBS) which are linked to the strong-coupling Mott limit. The phase diagram is obtained by quantum Monte Carlo (QMC) and continuous unitary transformations (CUTs). The CUT is performed in a two-step process: Nonperturbative graph-based CUTs are used in the Mott insulating phase to integrate out charge fluctuations. The resulting effective spin model is tackled by perturbative CUTs about the isolated dimer limit yielding the breakdown of the VBS by triplon condensation. We find three scenarios when varying the interaction for a fixed anisotropy of hopping amplitudes: (i) one direct phase transition from Néel to semimetal, (ii) two phase transitions VBS to Néel and Néel to semimetal, or (iii) a smooth crossover from VBS to the band insulator. Our results are consistent with the absence of spin-liquid phases in the whole phase diagram.

We present a complete theoretical treatment of Stark effects in bulk doped silicon, whose predictions are supported by experimental measurements. A multivalley effective mass theory, dealing nonperturbatively with valley-orbit interactions induced by a donor-dependent central cell potential, allows us to obtain a very reliable picture of the donor wave function within a relatively simple framework. Variational optimization of the $1s$ donor binding energies calculated with a new trial wave function, in a pseudopotential with two fitting parameters, allows an accurate match of the experimentally determined donor energy levels, while the correct limiting behavior for the electronic density, both close to and far from each impurity nucleus, is captured by fitting the measured contact hyperfine coupling between the donor nuclear and electron spin. We go on to include an external uniform electric field in order to model Stark physics: with no extra ad hoc parameters, variational minimization of the complete donor ground energy allows a quantitative description of the field-induced reduction of electronic density at each impurity nucleus. Detailed comparisons with experimental values for the shifts of the contact hyperfine coupling reveal very close agreement for all the donors measured (P, As, Sb, and Bi). Finally, we estimate field ionization thresholds for the donor ground states, thus setting upper limits to the gate manipulation times for single qubit operations in Kane-like architectures: the Si:Bi system is shown to allow for $A$ gates as fast as $\approx 10$ MHz.

Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of minerals that altogether constitute $\sim 95$ vol% of the Earth's lower mantle: ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate $\left({\mathrm{MgSiO}}_3\right)$ perovskite. Despite great strides made in the past decade, the existence of an intermediate-spin (IS) state in ferrous iron $\left({\mathrm{Fe}}^{2+}\right)$ (with total electron spin $S=1)$ and its possible role in the pressure-induced spin crossover in these lower-mantle minerals still remain controversial. Using density functional theory $+$ self-consistent Hubbard $U$ $\left(\text{DFT}+U_{\mathrm{sc}}\right)$ calculations, we investigate all possible types of IS states of ${\mathrm{Fe}}^{2+}$ in (Mg,Fe)O and (Mg,Fe)${\mathrm{SiO}}_3$ perovskite. Among the possible IS states in these minerals, the most probable IS state has an electronic configuration that significantly reduces the electron overlap and the iron nuclear quadrupole splitting (QS). These most probable IS states, however, are still energetically disfavored, and their QSs are inconsistent with Mössbauer spectra. We therefore conclude that IS ${\mathrm{Fe}}^{2+}$ is highly unlikely in the Earth's lower mantle.

We investigate the nature of excitons bound to $I_1$ basal-plane stacking faults $\left[(I_1,X)\right]$ in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i.e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the $(I_1,X)$ and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the $(I_1,X)$ recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of $210{\mathrm{cm}\mathrm{s}}^{-1}$ considering the nanowires mean diameter of 50 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than $6\times {10}^{16}$ ${\mathrm{cm}}^{-3}$. As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state.

The optical transitions and band alignments of $\text{PbTe}/{\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\text{Te}$ and $\text{PbSe}/{\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\text{Se}$ multi-quantum-well heterostructures were studied using temperature-modulated absorption spectroscopy and envelope function calculations. By taking advantage of the strain-induced splitting of conduction- and valence-band states at the different $L$ points of the Brillouin zone, a reliable determination of the band offsets is obtained, taking into account the biaxial strain of the quantum wells derived by x-ray diffraction. Through this analysis, the normalized conduction-band offsets are determined as $\Delta E_c/\Delta E_g=0.45\pm 0.05$ for the $\text{PbTe}/{\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\text{Te}$ system and $0.6\pm 0.1$ for the $\text{PbSe}/{\mathrm{Pb}}_{1-x}{\mathrm{Sr}}_x\text{Se}$ system for Sr contents up to 13%. Within the experimental precision, the band offsets are independent of temperatures from 20–300 K. With these parameters, precise modeling of the energy levels and optical transitions is achieved as required for optoelectronic device applications.

In this work we present a systematic study of the magnetic interactions within $3d$ transition-metal chains adsorbed on ${\mathrm{Cu}}_2\mathrm{N}$ and ${\mathrm{Cu}}_2\mathrm{O}$ monolayers grown on Cu(001). We are interested in the particular geometric adsorption configuration which gives rise, after relaxation, to the development of diatomic $\mathrm{TM}-X(X=\mathrm{N},\mathrm{O})$ chains. By using density functional theory (DFT), we calculate the energy difference between the ferromagnetic and antiferromagnetic intrachain configurations for Ti, V, Cr, Mn, Fe, and Co. Both substrates give rise, with minor differences, to the same magnetic trends, the only chains which are ferromagnetic after adsorption are Cr chains. By performing similar calculations in unsupported chains and introducing a tight-binding-model Hamiltonian based on physically reasonable assumptions we reproduce the magnetic trends obtained from the DFT calculations.

We investigate electron transport in disordered Hubbard chains contacted to macroscopic leads, via the nonequilibrium Green's function technique. We observe a crossover of currents and conductances at finite bias which depends on the relative strength of disorder and interactions. We provide a proof that the coherent potential approximation, a widely used method for treating disorder averages, fulfills particle conservation at finite bias with or without electron correlations. Finally, our results hint that the observed trends in conductance due to interactions and disorder also appear as signatures in the single-site entanglement entropy.

A quantum Monte Carlo method that combines the second-order many-body perturbation theory and Monte Carlo (MC) integration has been developed for correlation and correlation-corrected (quasiparticle) energy bands of one-dimensional solids. The sum-of-product expressions of correlation energy and self-energy are transformed, with the aid of a Laplace transform, into high-dimensional integrals, which are subject to a highly scalable MC integration with the Metropolis algorithm for importance sampling. The method can compute correlation energies of polyacetylene and polyethylene within a few $\text{m}{E}_{\text{h}}$ and quasiparticle energy bands within a few tenths of an eV. It does not suffer from the fermion sign problem and its description can be systematically improved by raising the perturbation order.

Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite ${\mathrm{BaCrO}}_3$ with a formal $3d^2$ configuration, the volume of which is apparently substantially enhanced by a strain due to ${\mathrm{SrTiO}}_3$ substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects leads to a metal-insulator transition and in-plane zigzag orbital ordering (OO) of alternating singly filled $d_{xz}+i{d}_{yz}$ and $d_{xz}-i{d}_{yz}$ orbitals, which results in a large orbital moment $M_L\approx -0.78{\mu }_B$ antialigned to the spin moment $M_S\approx 2\left|M_L\right|$ in Cr ions. Remarkably, this ordering also induces a considerable $M_L$ for apical oxygens. Our findings show metal-insulator and OO transitions, driven by an interplay among strain, correlation, and SOC, which is uncommon in $3d$ systems.

We investigate the influence of a Markovian environment on the dynamics of interacting spinful fermionic atoms in a lattice. To explore the physical phenomena occurring at short times, we develop a method based on a slave-spin representation of fermions that is amenable to the investigation of the dynamics of dissipative systems. We apply this approach to two different dissipative couplings that can occur in current experiments: a coupling via the local density and a coupling via the local double occupancy. We complement our study based on this method, with results obtained using the adiabatic elimination technique and with an exact study of a two-site model. We uncover that the decoherence is slowed down by increasing either the interaction strength or the dissipative coupling (the Zeno effect). We also find, for the coupling to the local double occupancy, that the final steady state can sustain single-particle coherence.

In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, $\gamma$, in an ensemble of InAs/GaAs quantum dots. We find that $\gamma$ is sensitive to the energetic overlap of the charged exciton spin states, which can be controlled through the application of an external magnetic field in the Faraday configuration. At zero applied field, electron-nuclear coupling induces additional dephasing beyond the radiative limit and $\gamma =230$ MHz ($0.95$ $\mu \mathrm{eV}$). Screening of the hyperfine interaction is achieved for an external field of $\approx 1$ T, resulting in $\gamma =172$ MHz ($0.71$ $\mu \mathrm{eV}$) limited only by spontaneous recombination along the dipole-allowed transition. These results are reproduced with a simple and intuitive model that captures the essential features of the electron hyperfine interaction and its influence on $\gamma$.

Tight-binding models for the recently observed surface electronic bands of ${\mathrm{SrTiO}}_3$ and ${\mathrm{KTaO}}_3$ are analyzed with a view to bringing out the relevance of momentum-space chiral angular momentum structures of both orbital and spin characters. The orbital and the accompanying spin angular momentum structures reveal complex linear and cubic dependencies in the momentum $\mathbf{k}$ (linear and cubic Rashba effects, respectively) in a band-specific manner. Analytical expressions for the cubic orbital and spin Rashba effects are derived by way of a unitary transformation technique we developed, and compared to numerical calculations. The cubic Rashba effect appears as in-plane modulations due to the $C_{4v}$ symmetry of the perovskite structure.

We consider the orbital magnetic properties of noninteracting charge carriers in graphene-based nanostructures in the low-energy regime. The magnetic response of such systems results both from bulk contributions and from confinement effects that can be particularly strong in ballistic quantum dots. First we provide a comprehensive study of the magnetic susceptibility $\chi$ of bulk graphene in a magnetic field for the different regimes arising from the relative magnitudes of the energy scales involved, i.e., temperature, Landau-level spacing, and chemical potential. We show that for finite temperature or chemical potential, $\chi$ is not divergent although the diamagnetic contribution ${\chi }_0$ from the filled valance band exhibits the well-known $-B^{-1/2}$ dependence. We further derive oscillatory modulations of $\chi$, corresponding to de Haas–van Alphen oscillations of conventional two-dimensional electron gases. These oscillations can be large in graphene, thereby compensating the diamagnetic contribution ${\chi }_0$ and yielding a net paramagnetic susceptibility for certain energy and magnetic field regimes. Second, we predict and analyze corresponding strong, confinement-induced susceptibility oscillations in graphene-based quantum dots with amplitudes distinctly exceeding the corresponding bulk susceptibility. Within a semiclassical approach we derive generic expressions for orbital magnetism of graphene quantum dots with regular classical dynamics. Graphene-specific features can be traced back to pseudospin interference along the underlying periodic orbits. We demonstrate the quality of the semiclassical approximation by comparison with quantum-mechanical results for two exemplary mesoscopic systems, a graphene disk with infinite mass-type edges, and a rectangular graphene structure with armchair and zigzag edges, using numerical tight-binding calculations in the latter case.