Resonant optical pumping of $f-f$ electronic transitions in the ${\mathrm{Dy}}^{3+}$ subsystem with femtosecond laser pulses in the multisublattice antiferromagnet ${\mathrm{DyFeO}}_3$ produces a strongly pronounced effect on the induced magnetization dynamics. Analyzing the polarization and spectral properties of the emitted THz radiation, we infer that the resonant pumping magnetizes the partially ordered ${\mathrm{Dy}}^{3+}$ ions on a femtosecond time scale with the induced longitudinal change of the magnetization reaching almost $1\%$. We also show that for laser photon energies close to the $f-f$ resonance of ${\mathrm{Dy}}^{3+}$ ions, a minimum in the efficiency of spin-wave excitation in the ${\mathrm{Fe}}^{3+}$ subsystem via the inverse Faraday effect is observed. This observation reveals that the resonant photo-induced magnetization in the ${\mathrm{Dy}}^{3+}$ subsystem and the off-resonant excitation of spin waves in the ${\mathrm{Fe}}^{3+}$ subsystem are intrinsically competing processes.

Superparamagnetic iron oxide nanoparticles with highly nonlinear magnetic behavior are attractive for biomedical applications like magnetic particle imaging and magnetic fluid hyperthermia. Such particles display interesting magnetic properties in alternating magnetic fields and here we document experiments that show differences between the magnetization dynamics of certain particles in frozen and melted states. This effect goes beyond the small temperature difference ($\mathrm{\Delta }T\sim 20{}^{\circ }\text{C}$) and we show the dynamics to be a mixture of Brownian alignment of the particles and Néel rotation of their moments occurring in liquid particle suspensions. These phenomena can be modeled in a stochastic differential equation approach by postulating log-normal distributions and partial Brownian alignment of an effective anisotropy axis. We emphasize that precise particle-specific characterization through experiments and nonlinear simulations is necessary to predict dynamics in solution and optimize their behavior for emerging biomedical applications including magnetic particle imaging.

Nonlocal pairing processes at the edge of a two-dimensional topological insulator in proximity to an $s$-wave superconductor are usually suppressed by helicity. However, the additional proximity of a ferromagnetic insulator can substantially influence the helical constraint and therefore open a new conduction channel by allowing for crossed Andreev reflection (CAR) processes. We show a one-to-one correspondence between CAR and the emergence of odd-frequency triplet superconductivity. Hence, nonlocal transport experiments that identify CAR in helical liquids yield smoking-gun evidence for unconventional superconductivity. Interestingly, we identify a setup—composed of a superconductor flanked by two ferromagnetic insulators—that allows us to favor CAR over electron cotunneling, which is known to be a difficult but essential task to be able to measure CAR.

The valence and core-level x-ray photoemission spectra (XPS), performed on an $\mathrm{UF}{\mathrm{e}}_2\mathrm{A}{\mathrm{l}}_{10}$ single crystal, were measured using the Al Kα radiation. The results of valence XPS show practically two separate regions of spectral intensity, one just at the Fermi level $\left(E_{\mathrm{F}}\right)$ and the other one being a wide content with its maximum at about $0.8\mathrm{eV}$ below $E_{\mathrm{F}}$. These give rise to two electronic configurations of the $5f$ states in the studied aluminide, itinerant and localized ones, i.e., their dual character. In such a situation the corresponding valence spectra, calculated within the local density approximation (LDA), well explain the former configuration, being responsible for a metallic behavior of the studied compound. Moreover, this behavior is confirmed clearly also by our results of magnetotransport measurements. On the other hand, the obtained magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power data support very well the local character of the $5f^2$-electron configuration of the ${\mathrm{U}}^{4+}$ ion in $\mathrm{UF}{\mathrm{e}}_2\mathrm{A}{\mathrm{l}}_{10}$ having the orthorhombic and cage-type crystal structure. Based on that configuration, the magnetic and thermal characteristics of the compound were modeled by the effective crystal field (CF) potential in the intermediate coupling scheme using initial parameters obtained by the angular overlap model (AOM). The obtained final CF parameters yielded the CF level scheme, composed of only singlets, proper for orthorhombic symmetry. Such a set of singlets reproduces in a satisfactory way both the strongly anisotropic temperature variations of the magnetic susceptibility, measured along the three main crystallographic directions, as well as the Schottky anomaly, evaluated using specific heat results of isomorphic $\mathrm{ThF}{\mathrm{e}}_2\mathrm{A}{\mathrm{l}}_{10}$ as a phonon reference. Also, the strongly anisotropic behavior of the Seebeck coefficient and its low temperature maxima observed for the compound studied here have been explained roughly by the CF effect.

Three-dimensional Dirac and Weyl semimetals exhibit a disorder-induced quantum phase transition between a semimetallic phase at weak disorder and a diffusive-metallic phase at strong disorder. Despite considerable effort, both numerically and analytically, the critical exponents $\nu$ and $z$ of this phase transition are not known precisely. Here we report a numerical calculation of the critical exponent $\nu =1.47\pm 0.03$ using a minimal single-Weyl node model and a finite-size scaling analysis of conductance. Our high-precision numerical value for $\nu$ is incompatible with previous numerical studies on tight-binding models and with one- and two-loop calculations in an $\epsilon$-expansion scheme. We further obtain $z=1.49\pm 0.02$ from the scaling of the conductivity with chemical potential.

A high-energy electron in condensed matter deposits energy by creation of electron-hole pairs whose density generally increases as the electron slows, reaching the order of ${10}^{20}$ eh/${\mathrm{cm}}^3$ near the end of its track. The subsequent interactions of the electrons and holes include nonlinear rate terms and transport as first hot and then thermalized carriers in the nanometer-scale radial dimension of the track. Charge separation and strong radial electric fields occur in a material such as CsI with contrasting diffusion rates of self-trapped holes and hot electrons. Eventual radiative recombination has a nonlinear relation to the primary electron energy because of these interactions. This so-called intrinsic nonproportionality of electron response limits the achievable energy resolution of a given scintillation radiation detector material. We use a system of coupled transport and rate equations to describe a pure host (three equations) and one dopant (four more equations per dopant). Applying it first to the experimentally well-characterized system of CsI and CsI:Tl in this work, we use results of picosecond absorption spectroscopy, interband $Z$-scan measurements of nonlinear rate constants, and other experiments and calculations to determine most of the more than 20 rate and transport coefficients required for modeling. The model is solved in a track environment approximated as cylindrical and is compared to the proportionality curve and total light yield of undoped CsI at temperatures of 295 and 100 K, as well as thallium dopant in CsI:Tl at 295 K. With this degree of validation, the space and time distributions of carriers and excitons, both untrapped and trapped, are examined within the model to gain an understanding of the main competitions controlling the nonproportionality of response.

We demonstrate tunability of the electron spin lifetime in Mn-doped GaAs by purely optical means. The observed behavior stems from a crossover of the electron spin relaxation rate with increasing excitation density, first decreasing due to the exchange interaction of Mn bound holes with Mn ions, and then increasing again as the valence band is populated and Bir-Aranov-Pikus relaxation sets in. On this account, we explain the complex spatial spin polarization profiles emerging from inhomogeneous optical excitation, which are the result of the combined action of this nonmonotonic spin relaxation characteristics and the intricate photocarrier decay dynamics.

Using a combination of scanning tunneling microscopy (STM) imaging and total-energy calculations within density functional theory, we evaluate a range of new and existing reconstruction models for Si nanoribbons that form on Ag(110). We propose two models that are consistent with experimental estimates of the Si coverage and the previously reported formation of Ag missing rows. The models yield STM images in excellent agreement with experiment, and are thermodynamically stable. This work provides clear evidence for a strongly bound Si-Ag reconstruction on Ag(110), and does not find any support for models based on silicene.

We study collective plasmon excitations and screening of pure and disordered single- and bilayer black phosphorus (BP) beyond the low energy continuum approximation. The dynamical polarizability of phosphorene is computed using a tight-binding model that properly accounts for the band structure in a wide energy range. Electron-electron interaction is considered within the random phase approximation. Damping of the plasmon modes due to different kinds of disorder, such as resonant scatterers and long-range disorder potentials, is analyzed. We further show that an electric field applied perpendicular to bilayer phosphorene can be used to tune the dispersion of the plasmon modes. For sufficiently large electric field, the bilayer BP enters in a topological phase with a characteristic plasmon spectrum, which is gaped in the armchair direction.

Using a nonperturbative functional renormalization group approach, we calculate the renormalized quasiparticle velocity $v\left(k\right)$ and the static dielectric function $\epsilon \left(k\right)$ of suspended graphene as functions of an external momentum $k$. Our numerical result for $v\left(k\right)$ can be fitted by $v\left(k\right)/v_F=A+Bln({\mathrm{\Lambda }}_0/k)$, where $v_F$ is the bare Fermi velocity, ${\mathrm{\Lambda }}_0$ is an ultraviolet cutoff, and $A=1.37, B=0.51$ for the physically relevant value ($e^2/v_F=2.2$) of the coupling constant. In contrast to calculations based on the static random-phase approximation, we find that $\epsilon \left(k\right)$ approaches unity for $k\to 0$. Our result for $v\left(k\right)$ agrees very well with a recent measurement by Elias et al. [Nat. Phys. 7, 701 (2011)].

We theoretically investigate the properties of two-dimensional topological insulator constrictions both in the integer and fractional regimes. In the presence of a perpendicular magnetic field, the constriction functions as a spin filter with near-perfect efficiency and can be switched by electric fields only. Domain walls between different topological phases can be created in the constriction as an interface between tunneling, magnetic fields, charge density wave, or electron-electron interaction dominated regions. These domain walls host non-Abelian bound states with fractional charge and spin and result in degenerate ground states with parafermions. If a proximity gap is induced bound states give rise to an exotic Josephson current with $8\pi$ periodicity.

The interplay between Mott and Anderson routes to localization in disordered interacting systems gives rise to different transitions and transport regimes. Here, we investigate the phase diagram at finite temperatures using dynamical mean-field theory combined with typical medium theory, which is an effective theory of the Mott-Anderson metal-insulator transition. We mainly focus on the properties of the coexistence region associated with the Mott phase transition. For weak disorder, the coexistence region is found to be similar to that in the clean case. However, as we increase disorder, Anderson localization effects are responsible for shrinking the coexistence region, and at sufficiently strong disorder (approximately equal to twice the bare bandwidth) it drastically narrows, the critical temperature $T_c$ abruptly goes to zero, and we observe a phase transition in the absence of a coexistence of the metallic and insulating phases. In this regime, the effects of interaction and disorder are found to be of comparable importance for charge localization.

Concepts of information theory are increasingly used to characterize collective phenomena in condensed matter systems, such as the use of entanglement entropies to identify emergent topological order in interacting quantum many-body systems. Here, we employ classical variants of these concepts, in particular Rényi entropies and their associated mutual information, to identify topological order in classical systems. Like for their quantum counterparts, the presence of topological order can be identified in such classical systems via a universal, subleading contribution to the prevalent volume and boundary laws of the classical Rényi entropies. We demonstrate that an additional subleading $O\left(1\right)$ contribution generically arises for all Rényi entropies $S^{\left(n\right)}$ with $n\ge 2$ when driving the system towards a phase transition, e.g., into a conventionally ordered phase. This additional subleading term, which we dub connectivity contribution, tracks back to partial subsystem ordering and is proportional to the number of connected parts in a given bipartition. Notably, the Levin-Wen summation scheme, typically used to extract the topological contribution to the Rényi entropies, does not fully eliminate this additional connectivity contribution in this classical context. This indicates that the distillation of topological order from Rényi entropies requires an additional level of scrutiny to distinguish topological from nontopological $O\left(1\right)$ contributions. This is also the case for quantum systems, for which we discuss which entropies are sensitive to these connectivity contributions. We showcase these findings by extensive numerical simulations of a classical variant of the toric code model, for which we study the stability of topological order in the presence of a magnetic field and at finite temperatures from a Rényi entropy perspective.

Electrons in certain two-dimensional crystals possess a pseudospin degree of freedom associated with the existence of two inequivalent valleys in the Brillouin zone. If, as in monolayer ${\mathrm{MoS}}_2$, inversion symmetry is broken and time-reversal symmetry is present, equal and opposite amounts of $k$-space Berry curvature accumulate in each of the two valleys. This is conveniently quantified by the integral of the Berry curvature over a single valley—the valley Hall conductivity. We generalize this definition to include contributions from disorder described with the supercell approach, by mapping (“unfolding”) the Berry curvature from the folded Brillouin zone of the disordered supercell onto the normal Brillouin zone of the pristine crystal, and then averaging over several realizations of disorder. We use this scheme to study from first principles the effect of sulfur vacancies on the valley Hall conductivity of monolayer ${\mathrm{MoS}}_2$. In dirty samples the intrinsic valley Hall conductivity receives gating-dependent corrections that are only weakly dependent on the impurity concentration, consistent with side-jump scattering and the unfolded Berry curvature can be interpreted as a $k$-space resolved side jump. At low impurity concentrations skew scattering dominates, leading to a divergent valley Hall conductivity in the clean limit. The implications for the recently observed photoinduced anomalous Hall effect are discussed.

The superconducting critical temperature, $T_{\mathrm{c}}$, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a ${}^{77}\mathrm{Se},{}^7\mathrm{Li}$, and ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded ${\mathrm{Li}}_x{\left({\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2\right)}_y{\mathrm{Fe}}_{2-z}{\mathrm{Se}}_2$ with a nearly optimal $T_{\mathrm{c}}=45$ K. The absence of any shift in the ${}^7\mathrm{Li}$ and ${}^1\mathrm{H}$ NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent ${}^7\mathrm{Li}$ and ${}^1\mathrm{H}$ spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, the strong temperature dependence of ${}^{77}\mathrm{Se}$ NMR shift and spin-lattice relaxation rate, $1{/}^{77}{T}_1$, is attributed to the holelike bands close to the Fermi energy. $1{/}^{77}{T}_1$ shows no additional anisotropy that would account for the onset of electronic nematic order down to $T_{\mathrm{c}}$. Similarly, no enhancement in $1{/}^{77}{T}_1$ due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence $1{/}^{77}{T}_1\propto T^{4.5}$ still complies with the Cooper pairing mediated by spin fluctuations.

The system $\mathrm{HoA}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ has recently been found to exhibit a large magnetoelectric effect. To understand the mechanism, macroscopic and atomic level properties of $\mathrm{HoA}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ were explored by temperature and magnetic field dependent heat capacity measurements, pressure and temperature dependent x-ray diffraction measurements, as well as temperature and magnetic field dependent x-ray absorption fine structure measurements. The experimental work was complemented by density functional theory calculations. An anomalous change in the structure is found in the temperature range where large magnetoelectric effects occur. No significant structural change or distortion of the $\mathrm{Ho}{\mathrm{O}}_6$ polyhedra is seen to occur with magnetic field. However, the magnetic field dependent structural measurements reveal enhanced correlation between neighboring $\mathrm{Ho}{\mathrm{O}}_6$ polyhedra. This observed response is seen to saturate near 3 T. A qualitative atomic level description of the mechanism behind the large electric polarization induced by magnetic fields in the general class of $\mathrm{RA}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ systems $(R=$ rare earth) is developed.

Temperature dependencies of the electrical resistivity, magnetic susceptibility, and heat capacity have been obtained in the cubic phase of ${\mathrm{TbGe}}_{2.85}$ (the ${\mathrm{AuCu}}_3$ structure), synthesized at high pressure. The macroscopic measurements indicate that a charge density wave is formed below 145 K and an antiferromagnetic ordering is realized below 19 K. Hyperfine interaction data obtained with the time differential perturbed angular correlation method with ${}^{111}\mathrm{Cd}$ probes inserted in the ${\mathrm{TbGe}}_{2.85}$ lattice suggest that the charge density wave is incommensurate in the temperature region 19–145 K, but becomes commensurate below 19 K. The neutron diffraction reveals a complex antiferromagnetic spiral structure in the magnetically ordered phase. We discuss relations between the charge density wave and helical ordering in ${\mathrm{TbGe}}_{2.85}$ and ${\mathrm{TbPd}}_3$.

We have investigated thermoelectric properties of a three-dimensional Rashba system BiTeI. Magnetic-field dependences of the Seebeck effect and Nernst effect show qualitative changes with the Fermi level passing through the bulk Dirac point, indicating that thermoelectric effects can be a good experimental probe for the Fermi surface topology. The quantum oscillations are observed in the thermoelectric effect of BiTeI under a magnetic field, which are also dependent on the Fermi-level positions and consistent with the energy derivative of the three-dimensional density of states in the Rashba system.

We show that Weyl semimetals with broken time-reversal symmetry can host chiral electromagnetic waves. The magnetization that results in a momentum-space separation of a pair of opposite chirality Weyl nodes is also responsible for the nonzero gyrotropy parameter in the system. It is then shown that a chiral electromagnetic wave can propagate in a region of space where the gyrotropy parameter changes sign. Such waves are analogs of quantum Hall edge states for photons.