The effect of an applied electric field and the effect of charging are investigated on the magnetic anisotropy (MA) of various stable two-dimensional (2D) crystals such as graphene, ${\mathrm{FeCl}}_2$, graphone, fluorographene, and ${\mathrm{MoTe}}_2$ using first-principles calculations. We found that the magnetocrystalline anisotropy energy of Co-on-graphene and Os-doped-${\mathrm{MoTe}}_2$ systems change linearly with electric field, opening the possibility of electric field tuning MA of these compounds. In addition, charging can rotate the easy-axis direction of Co-on-graphene and Os-doped-${\mathrm{MoTe}}_2$ systems from the out-of-plane (in-plane) to in-plane (out-of-plane) direction. The tunable MA of the studied materials is crucial for nanoscale electronic technologies such as data storage and spintronics devices. Our results show that controlling the MA of the mentioned 2D crystal structures can be realized in various ways, and this can lead to the emergence of a wide range of potential applications where the tuning and switching of magnetic functionalities are important.

Electronic spin and orbital $\left(dd\right)$ excitation spectra of $({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}$ )$\left({\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}\right){\mathrm{Cu}}_3{\mathrm{O}}_y$ samples are measured by resonant inelastic x-ray scattering (RIXS). In this compound, $T_c$ of samples with identical hole dopings is strongly affected by the Ca/Ba substitution $x$ due to subtle variations in the lattice constants, while crystal symmetry and disorder as measured by linewidths are $x$ independent. We examine two extreme values of $x$ and two extreme values of hole-doping content $y$ corresponding to antiferromagnetic and superconducting states. The $x$ dependence of the spin-mode energies is approximately the same for both the antiferromagnetic and superconducting samples. This clearly demonstrates that RIXS is sensitive to the superexchange $J$ even in doped samples. A positive correlation between $J$ and the maximum of $T_c$ at optimal doping $\left(T_c^{max}\right)$ is observed. We also measured the $x$ dependence of the $d_{xy}\to d_{x^2-y^2}$ and $d_{xz/yz}\to d_{x^2-y^2}$ orbital splittings. We infer that the effect of the unresolved $d_{3z^2-r^2}\to d_{x^2\to y^2}$ excitation on $T_c^{max}$ is much smaller than the effect of $J$. There appears to be dispersion in the $d_{xy}\to d_{x^2-y^2}$ peak of up to 0.05 eV. Our fitting furthermore indicates an asymmetric dispersion for the $d_{xz/yz}\to d_{x^2-y^2}$ excitation. A peak at $\sim 0.8$ eV is also observed and attributed to a $dd$ excitation in the chain layer.

We present a continuous-time Monte Carlo impurity solver for multiorbital impurity models which combines a strong-coupling hybridization expansion and a weak-coupling expansion in the Hund's coupling parameter $J$. This double-expansion approach allows to treat the dominant density-density interactions $U$ within the efficient segment representation. We test the approach for a two-orbital model with static interactions, and then explain how the double expansion allows to simulate models with frequency dependent $U\left(\omega \right)$ and $J\left(\omega \right)$. The method is used to investigate spin-state transitions in a toy model for fullerides, with repulsive bare $J$ but attractive screened $J$.

We study the practicability of achieving quantum anomalous Hall (QAH) effect with a field-tunable Chern number in a magnetically doped, topologically trivial insulating thin film. Specifically, in a candidate material, $\mathrm{TlBi}({\mathrm{S}}_{1-\delta }{\mathrm{Se}}_{\delta }){}_2$, we demonstrate that the QAH phases with different Chern numbers can be achieved by means of tuning the exchange field strength or the sample thickness near the $Z_2$ topological critical point. Our physics scenario successfully reduces the necessary exchange coupling strength for a targeted Chern number. This QAH mechanism differs from the traditional QAH picture with a magnetic topological insulating thin film, where the “surface” states must involve and sometimes complicate the realization issue. Furthermore, we find that a given Chern number can also be tuned by a perpendicular electric field, which naturally occurs when a substrate is present.

The (001) and (111) surface structures of the $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ (LMO) spinel are examined using density functional theory (DFT) calculations within the generalized gradient approximation $\left(\mathrm{GGA}\right)+U$ approach. In order to clarify discrepancies in the literature for previous DFT calculations of these surfaces, we first carefully study the effects of surface termination/reconstruction, slab construction, and relaxation schemes, as well as magnetic ordering and $U$ values for Mn on the calculated surface energies of LMO. We explain these discrepancies and show that the relaxation scheme and surface reconstruction play the key role in determining the relative stability of (001) and (111) surfaces. We have further analyzed the thermodynamic stability of LMO surfaces as a function of oxygen and lithium chemical potentials. We have found that the ratio of (001) to (111) surface energies is $\sim 1.09$ up to the oxygen chemical potential corresponding to $\sim 800\mathrm{K}$. This ratio favors the formation of truncated-octahedron shaped LMO particles dominated by Li-terminated reconstructed (111) facets with no surface Mn, which can help suppress Mn dissolution. Higher temperatures or closed systems (with no exchange of material with surroundings) favor the formation of (001) dominated particles. The observation of a wide spectrum of polyhedral shapes between (001)- and (111)-dominated LMO particles in experiments can be explained by the narrow range of surface energies and their sensitivity to synthesis conditions.

Using quantum Monte Carlo, we study the nonequilibrium transport of magnetization in large open strongly correlated quantum spin-$\frac{1}{2}$ systems driven by purely dissipative processes that conserve the uniform or staggered magnetization, disregarding unitary Hamiltonian dynamics. We prepare both a low-temperature Heisenberg ferromagnet and an antiferromagnet in two parts of the system that are initially isolated from each other. We then bring the two subsystems in contact and study their real-time dissipative dynamics for different geometries. The flow of the uniform or staggered magnetization from one part of the system to the other is described by a diffusion equation that can be derived analytically.

Chemical substitutions are used commonly to tune a magnetic transition to zero temperature, but the resulting non-Fermi-liquid (NFL) behavior is nonuniversal. We have used nuclear quadrupole resonance to probe microscopically the response of a prototypical quantum critical metal ${\mathrm{CeCoIn}}_5$ to substitutions of small amounts of Sn and Cd for In. These substituents induce very different local electronic environments as observed by site-dependent spin lattice relaxation rates $1/T_1$ that influence the NFL behavior. The effects found here illustrate the need for care in interpreting NFL properties determined by macroscopic measurements.

We have performed time-resolved (TR) core-level and valence-band angle-resolved (AR) photoemission spectroscopy (PES) to investigate ultrafast dynamics of an electron-doped topological insulator ${\text{Cu}}_{0.17}{\text{Bi}}_2{\text{Se}}_3$. The Bi $5d_{5/2}$ line was composed of a single peak and exhibited broadening upon both heating and pumping, which we interpreted as a change of phonon temperature ($T_p$), in the surface region. The electronic dynamics and electron temperature ($T_e$), on the other hand, were determined with near-$E_{\mathrm{F}}$ TRARPES. The transient temperature deduced from core-level TRPES shows a similar behavior with $T_e$ deduced from near-$E_{\mathrm{F}}$ TRARPES. This similar behavior of $T_p$ and $T_e$ can be reproduced not by a simple two-temperature model but by a modified one, although we cannot exclude a possibility that core-level broadening also reflects $T_e$.

We have investigated the role of $\mathrm{M}{\mathrm{n}}^{3+}$ Jahn-Teller distortion in driving ferromagnetism in the $\mathrm{LaM}{\mathrm{n}}_{1\text{−}x}\mathrm{S}{\mathrm{c}}_x{\mathrm{O}}_3$ series. The replacement of Mn by Sc in $\mathrm{LaMn}{\mathrm{O}}_3$ decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the $\mathrm{LaSc}{\mathrm{O}}_3$ sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in $\mathrm{LaMn}{\mathrm{O}}_3$ into a nearly regular one in $\mathrm{LaSc}{\mathrm{O}}_3$. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc $K$ edges reveal the stability of both Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_6$ octahedron and nearly regular $\mathrm{Sc}{\mathrm{O}}_6$ octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_6$ octahedra spatially ordered as in $\mathrm{LaMn}{\mathrm{O}}_3$ and nearly regular $\mathrm{Sc}{\mathrm{O}}_6$ octahedra. This result contrasts with the pseudocubic phase and the appearance of regular $\mathrm{Mn}{\mathrm{O}}_6$ octahedra in $\mathrm{LaM}{\mathrm{n}}_{1\text{−}x}\mathrm{G}{\mathrm{a}}_x{\mathrm{O}}_3$ for $x>0.5$. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the $\mathrm{M}{\mathrm{n}}^{3+}$ atom is allocated. Besides, a ferromagnetic ground state is observed for $x>0.5$ in both series independently of the presence (or not) of Jahn-Teller distortions around $\mathrm{M}{\mathrm{n}}^{3+}$, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in $\mathrm{LaM}{\mathrm{n}}_{1\text{−}x}{B}_x{\mathrm{O}}_3$ $(B=\text{Sc or Ga})$.

For the simulation of equilibrium states and finite-temperature response functions of strongly correlated quantum many-body systems, we compare the efficiencies of two different approaches in the framework of the density matrix renormalization group (DMRG). The first is based on matrix product purifications. The second, more recent one, is based on so-called minimally entangled typical thermal states (METTS). For the latter, we highlight the interplay of statistical and DMRG truncation errors, discuss the use of self-averaging effects, and describe schemes for the computation of response functions. For critical as well as gapped phases of the $\text{spin}-1/2$ XXZ chain and the one-dimensional Bose-Hubbard model, we assess the computation costs and accuracies of the two methods at different temperatures. For almost all considered cases, we find that, for the same computation cost, purifications yield more accurate results than METTS—often by orders of magnitude. The METTS algorithm becomes more efficient only for temperatures well below the system's energy gap. The exponential growth of the computation cost in the evaluation of response functions limits the attainable time scales in both methods and we find that in this regard, METTS do not outperform purifications.

We combine ab initio density functional theory calculations with the equivalent cores approximation to determine core-level binding-energy shifts at phosphorus sites caused by oxidation of phosphorene. We find that presence of oxygen increases the core-level binding energies of P atoms and expect binding-energy shifts of up to 6 eV in highly defective geometries. We have identified likely binding geometries of oxygen that help to interpret the observed core-level photoemission spectra in samples at different stages of oxidation and allow us to determine the fractions of specific local geometries.

We present an analysis of electron transport through two weakly coupled precision-placed phosphorus donors in silicon. In particular, we examine the (1,1)$\leftrightarrow$(0,2) charge transition where we predict a type of current blockade driven entirely by the nuclear spin dynamics. Using this nuclear spin blockade mechanism, we devise a protocol to read out the state of single nuclear spins using electron-transport measurements only. We extend our model to include realistic effects such as Stark shifted hyperfine interactions and multidonor clusters. In the case of multidonor clusters we show how nuclear spin blockade can be alleviated, allowing for low magnetic field electron-spin measurements.

The second-order ordering transition of the ${\mathrm{AuAgZn}}_2$ alloy has been studied by coherent x-ray scattering. Within a few degrees above the critical temperature $T_c$, equilibrium critical fluctuations are observed together with some pretransitional local ordering connected to sample defects. The speckles observed correspond to heterodyne interference between local ordering and fluctuations and show a mixed static and dynamical behavior in a narrow domain of a few tenths of degree above $T_c$. The dynamical behavior is shown to correspond to the critical slowing down of the fluctuations in the vicinity of the transition (model “A” of Hohenberg and Halperin [Rev. Mod. Phys. 49, 435 (1977)]). A rough comparison can be carried out with the classical diffusion models. Some improvements of the method are discussed.

We investigate properties of $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (122) single crystals upon gold doping, which is the transition metal with the highest atomic weight. The Au substitution into the FeAs-planes of 122 crystal structure (Au-122) is only possible up to a small amount of $\sim 3\%$. We find that $5d$ is more effective in reducing magnetism in 122 than its counter $3d$ Cu, and this relates to superconductivity. We provide evidence of short-range magnetic fluctuations and local lattice inhomogeneities that may prevent strong percolative superconductivity in $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{A}{\mathrm{u}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$.

The nonequilibrium dynamics of a system that is located in the vicinity of a quantum critical point is affected by the critical slowing down of order-parameter correlations with the potential for novel out-of-equilibrium universality. After a quantum quench, i.e., a sudden change of a parameter in the Hamiltonian, such a system is expected to almost instantly fall out of equilibrium and undergo aging dynamics, i.e., dynamics that depends on the time passed since the quench. Investigating the quantum dynamics of an $N$-component ${\phi }^4$ model coupled to an external bath, we determine this universal aging and demonstrate that the system undergoes a coarsening, governed by a critical exponent that is unrelated to the equilibrium exponents of the system. We analyze this behavior in the large-$N$ limit, which is complementary to our earlier renormalization-group analysis, allowing in particular the direct investigation of the order-parameter dynamics in the symmetry-broken phase and at the upper critical dimension. By connecting the long-time limit of fluctuations and response, we introduce a distribution function that shows that the system remains nonthermal and exhibits quantum coherence even on long time scales.

Finite-temperature ($T$) properties of a Kitaev model defined on a honeycomb lattice are investigated by a quantum Monte Carlo simulation, from the viewpoint of fractionalization of quantum $S=1/2$ spins into two types of Majorana fermions, itinerant and localized. In this system, the entropy is released successively at two well-separated $T$ scales, as a clear indication of the thermal fractionalization. We show that the high-$T$ crossover, which is driven by itinerant Majorana fermions, is closely related with the development of nearest-neighbor spin correlations. On the other hand, the low-$T$ crossover originates in thermal fluctuations of fluxes composed of localized Majorana fermions, by which the spectrum of itinerant Majorana fermions is significantly disturbed. As a consequence, in the intermediate-$T$ range between the two crossovers, the system exhibits $T$-linear behavior in the specific heat and coherent transport of Majorana fermions, which are unexpected for the Dirac semimetallic spectrum in the low-$T$ limit. We also show that the flux fluctuations tend to open an energy gap in the Majorana spectrum near the gapless-gapped phase boundary. Our results indicate that the fractionalization is experimentally observable in the specific heat, spin correlations, and transport properties.

We investigated the composition-dependent photocarrier dynamics in $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSn}{\left({\mathrm{S}}_x\mathrm{S}{\mathrm{e}}_{1-x}\right)}_4$ (CZTSSe) single crystals using various types of steady-state and time-resolved optical spectroscopy. Photoluminescence spectroscopy shows that the band-tail states formed below the band edge decrease monotonically with increasing Se content. THz time-resolved spectroscopy clarifies that an increase in the Se content leads to a shorter lifetime of the free photocarriers. A trade-off between the composition-dependent band-tail density and the free-carrier lifetime occurs in CZTSSe single crystals. Our experimental results provide insights into the physics behind the low and composition-dependent conversion efficiency of CZTSSe-based solar cells.

We analyze in detail the energy transport of surface phonon polaritons propagating in a chain of spheroidal polar nanoparticles with both longitudinal and transversal polarizations. Explicit and closed-form expressions for the dispersion relation and propagation length are derived and used to determine the values of the nanoparticle polarizability and the interparticle distance that maximize the polariton propagation length. The thermal conductance in the ballistic regime and the thermal conductivity in the diffusive one are also determined and examined as a function of the geometry of the nanoparticles and their temperature. For a chain of cigar-shaped SiC nanoparticles in contact, an aspect ratio of 5, and surrounded by air; it is shown that: (i) The surface phonon polaritons propagate a distance of $10\mu \mathrm{m}$ along a chain of 100 nanoparticles. This propagation length is one order of magnitude longer than that for spherical nanoparticles. (ii) The polariton thermal conductivity is comparable with the one of air in a wide range of temperatures and is $41\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 500 K. (iii) The polariton thermal conductance increases with the temperature and at 500 K is $44\mathrm{pW}{\mathrm{K}}^{-1}$, which represents $9\%$ of the quantum of thermal conductance. In view of the ultralow phonon thermal conductivity of a chain of polar nanoparticles in contact and their high surface area-to-volume ratios, the proposed theoretical model and obtained results are expected to be useful to experimentally quantify the energy transport of surface phonon polaritons propagating along these nanostructures.

Clean two-dimensional electron systems in GaAs/AlGaAs heterostructures exhibit anisotropic collective phases, the quantum Hall nematics, at high Landau-level occupancy and low temperatures. An as yet unknown native symmetry-breaking potential consistently orients these phases relative to the crystalline axes of the host material. Here we report an extensive set of measurements examining the role of the structural symmetries of the heterostructure in determining the orientation of the nematics. In single quantum well samples we find that neither the local symmetry of the confinement potential nor the distance between the electron system and the sample surface dictates the orientation of the nematic. In remarkable contrast, for two-dimensional electrons confined at a single heterointerface between GaAs and AlGaAs, the nematic orientation depends on the depth of the two-dimensional electron system beneath the sample surface.