Results of muon spin relaxation $\left(\mu \mathrm{SR}\right)$ and neutron powder diffraction measurements on a reentrant superconductor $\mathrm{Eu}\left({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}\right){}_2{\mathrm{As}}_2$ are presented. $\mathrm{Eu}\left({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}\right){}_2{\mathrm{As}}_2$ exhibits superconductivity at $T_{\mathrm{c}\mathrm{on}}\approx 22.5$ K competing with long range ordered ${\mathrm{Eu}}^{+2}$ moments below $\approx 18$ K. A reentrant behavior (manifested by nonzero resistivity in the temperature range 10–17.5 K) results from an exquisite competition between the superconductivity and magnetic order. The zero field $\mu \mathrm{SR}$ data confirm the long range magnetic ordering below $T_{\mathrm{Eu}}=18.7\left(2\right)$ K. The transition temperature is found to increase with increasing magnetic field in longitudinal field $\mu \mathrm{SR}$ which along with the neutron diffraction results, suggests the transition to be ferromagnetic. The neutron diffraction data reveal a clear presence of magnetic Bragg peaks below $T_{\mathrm{Eu}}$ which could be indexed with propagation vector $\mathbf{k}=(0,0,0)$, confirming a long range magnetic ordering in agreement with $\mu \mathrm{SR}$ data. Our analysis of the magnetic structure reveals an ordered magnetic moment of $6.29\left(5\right){\mu }_{\mathrm{B}}$ (at 1.8 K) on the Eu atoms and they form a ferromagnetic structure with moments aligned along the $c$ axis. No change in the magnetic structure is observed in the reentrant or superconducting phases and the magnetic structure remains same for $1.8\text{K}\le T\le T_{\mathrm{Eu}}$. No clear evidence of structural transition or Fe moment ordering was found.

Experimentally detected ultrafast spin avalanches spreading in crystals of molecular (nano)magnets [Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)] have recently been explained in terms of magnetic detonation [Modestov et al., Phys. Rev. Lett. 107, 207208 (2011)]. Here magnetic detonation structure is investigated by taking into account transport processes of the crystals such as thermal conduction and volume viscosity. The transport processes result in smooth profiles of the most important thermodynamical crystal parameters, temperature, density, and pressure, all over the magnetic detonation front, including the leading shock, which is one of the key regions of magnetic detonation. In the case of zero volume viscosity, thermal conduction leads to an isothermal discontinuity instead of the shock, for which temperature is continuous while density and pressure experience jump. It is also demonstrated that the thickness of the magnetic detonation front may be controlled by applying the transverse-magnetic field, which is important for possible experimental observations of magnetic detonation.

Recent works on hard spheres in the limit of infinite dimensions revealed that glass states, envisioned as metabasins in configuration space, can break up in a multitude of separate basins at low enough temperature or high enough pressure, leading to the emergence of new kinds of soft-modes and unusual properties. In this paper we study by perturbative renormalization group techniques the critical properties of this transition, which has been discovered in disordered mean-field models in the 1980s. We find that the upper-critical dimension $d_u$, above which mean-field results hold, is strictly larger than six and apparently nonuniversal, i.e., system dependent. Below $d_u$, we do not find any perturbative attractive fixed point (except for a tiny region of the one-step replica symmetry breaking parameter), thus showing that the transition in three dimensions either is governed by a nonperturbative fixed point unrelated to the Gaussian mean-field one or becomes first order or does not exist. We also discuss possible relationships with the behavior of spin glasses in a field.

Step-induced antiferromagnetic (AFM) uniaxial anisotropy and its effects on the exchange coupling have been systematically investigated in the epitaxial $\mathrm{Fe}/\mathrm{CoO}$ bilayers on a $\mathrm{MgO}\left(001\right)$ vicinal surface. X-ray magnetic linear dichroism measurements proved that the atomic steps induced a strong in-plane AFM uniaxial anisotropy in the CoO film. We found that the thermal activation induced in-plane 90° switching of CoO AFM in-plane spins. The competition among the step-induced AFM anisotropy, the interface exchange coupling, and thermal activation generate a novel multiple in-plane spin reorientation transition of the Fe magnetization, which can further provide new insights on the exchange coupling in FM/AFM systems.

We propose an environment recycling scheme to speed up a class of tensor network algorithms that produce an approximation to the ground state of a local Hamiltonian by simulating an evolution in imaginary time. Specifically, we consider the time-evolving block decimation (TEBD) algorithm applied to infinite systems in 1D and 2D, where the ground state is encoded, respectively, in a matrix product state (MPS) and in a projected entangled-pair state (PEPS). An important ingredient of the TEBD algorithm (and a main computational bottleneck, especially with PEPS in 2D) is the computation of the so-called environment, which is used to determine how to optimally truncate the bond indices of the tensor network so that their dimension is kept constant. In current algorithms, the environment is computed at each step of the imaginary time evolution, to account for the changes that the time evolution introduces in the many-body state represented by the tensor network. Our key insight is that close to convergence, most of the changes in the environment are due to a change in the choice of gauge in the bond indices of the tensor network, and not in the many-body state. Indeed, a consistent choice of gauge in the bond indices confirms that the environment is essentially the same over many time steps and can thus be re-used, leading to very substantial computational savings. We demonstrate the resulting approach in 1D and 2D by computing the ground state of the quantum Ising model in a transverse magnetic field.

The electronic structure of ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ was investigated using x-ray emission and absorption spectroscopy and by theoretical calculations employing two approaches: the local density approximation (LDA) and a combination of LDA with the cluster extension of dynamical mean-field theory (LDA+DMFT). The evolution of the spectral properties with the strength of electronic correlations is analyzed. We show that for moderate values of on-site Coulomb repulsion $U$ and intra-atomic Hund's rule exchange $J_H,{\mathrm{Li}}_2{\mathrm{RuO}}_3$ is in an orbital-selective strongly correlated state in the sense that a part of the $t_{2g}$ manifold (i.e., $xz/yz)$ behaves as local atomic orbitals susceptible to Hubbard correlations, while the remaining $\left(xy\right)$ orbitals must be described as bond-centered molecular orbitals. Both theoretical approaches succeed in explaining the x-ray data, and a comparison of the theoretical and experimental spectra provides a reasonable estimate of the possible correlation strength $\left(U\right)$ and Hund's coupling $\left(J_H\right)$ in ${\mathrm{Li}}_2{\mathrm{RuO}}_3$.

We theoretically explore through systematic multiscale ab initio and Monte Carlo calculations how the surface magnetism of a ferromagnetic surface can be fine-tuned by nonmagnetic organic molecules containing a single $\pi$ bond. We demonstrate that a magnetic hardening or softening can be induced depending on the electronegativity of the heteroatom or when the $\pi$-bond “bridges” the magnetic surface atoms. Finally, the Monte Carlo simulations revealed taylored macroscopic hysteresis loops corresponding to soft and hard molecule-surface magnets.

We investigate the possibility of band structure engineering in the recently predicted 2D layered form of blue phosphorus via an electric field $\left({\mathrm{E}}_z\right)$ applied perpendicular to the layer(s). Using density functional theory, we study the effect of a transverse electric field in monolayer as well as three differently stacked bilayer structures of blue phosphorus. We find that for ${\mathrm{E}}_z>0.2$ V/Å the direct energy gap at the $\Gamma$ point, which is much larger than the default indirect band gap of mono- and bilayer blue phosphorus, decreases linearly with the increasing electric field, becomes comparable to the default indirect band gap at ${\mathrm{E}}_z\approx 0.45\left(0.35\right)$ V/Å for monolayer (bilayers), and decreases further until the semiconductor to metal transition of 2D blue phosphorus takes place at ${\mathrm{E}}_z\approx 0.7\left(0.5\right)$ V/Å for monolayer (bilayers). Calculated values of the electron and hole effective masses along various high symmetry directions in the reciprocal lattice suggests that the mobility of charge carriers is also influenced by the applied electric field.

X-ray photoemission spectra generally exhibit satellite features beyond the main peak due to many-body excitations. However, the satellites associated with charge-transfer excitations in correlated materials have proved difficult to calculate from first principles and their interpretation has been controversial. Here we show that these satellites can be attributed to local density fluctuations in response to a suddenly created core hole. Our approach is based on a cumulant representation of the core-hole Green's function with a real-time, time-dependent density functional theory calculation of the cumulant. This approach includes effects that cannot be accounted for by cluster methods and yields a direct real-space, real-time interpretation. Illustrative results for ${\mathrm{TiO}}_2$ and NiO are in good agreement with XPS experiment.

Multiple zero-energy Majorana fermions (MFs) with spatially overlapping wave functions can survive only if their splitting is prevented by an underlying symmetry. Here we show that, in quasi-one-dimensional (Q1D) time-reversal-invariant topological superconductors (class DIII), a realistic model for superconducting lithium molybdenum purple bronze ${(\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17})$ and certain families of organic superconductors, multiple Majorana-Kramers pairs with strongly overlapping wave functions persist at zero energy even in the absence of an easily identifiable symmetry. We find that similar results hold in the case of Q1D semiconductor-superconductor heterostructures (class D) with $t_{\perp }\ll t$, where $t_{\perp }$ and $t$ are the transverse and longitudinal hoppings, respectively. Our results, explained in terms of special properties of the Hamiltonian and wave functions, underscore the importance of hidden accidental symmetries in topological superconductors.

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study the bilayer system of ${\mathrm{LaCoO}}_3$ grown along the $\left(111\right)$ direction on ${\mathrm{SrTiO}}_3$. The LDA calculations show that there are two nearly flat bands located at the top and bottom of $e_g$ bands of Co atoms with the Fermi level crossing the lower one. After including both the spin-orbit coupling and the Coulomb interaction in the LDA+G method, we find that the interplay between spin-orbit coupling and Coulomb interaction stabilizes a very robust ferromagnetic insulator phase with the nonzero Chern number indicating the possibility of realizing the quantum anomalous Hall effect in this system.

A physically transparent and mathematically simple semiclassical model is employed to examine dynamics in the central-spin problem. The results reproduce previous findings obtained by various quantum approaches and, at the same time, provide information on the electron spin dynamics and Berry's phase effects over a wider range of experimentally relevant parameters than available previously. This development is relevant to dynamics of bound magnetic polarons and spin dephasing of an electron trapped by an impurity or a quantum dot, and coupled by a contact interaction to neighboring localized magnetic impurities or nuclear spins. Furthermore, it substantiates the applicability of semiclassical models to simulate dynamic properties of spintronic nanostructures with a mesoscopic number of spins.

Ultrathin bismuth (Bi) layers on $\text{Si(111)-7}\times 7$ undergo a structural phase transformation with reducing the number of atomic layers at 3 bilayers (BL). We investigate the phonon dynamics of the Bi films close to the phase transformation by pump-probe reflectivity measurements. Coherent $A_{1g}$ and $E_g$ phonons at 3 and 2 THz are clearly observed for the Bi layers with thicknesses down to 3 BL, confirming their rhombohedral crystalline structure. The $A_{1g}$ frequency exhibits an abrupt redshift and splits into two components at 3 BL, which are attributed to the vertical motions of Bi atoms localized at the surface and subsurface bilayers. The $E_g$ frequency, by contrast, shows a gradual blueshift with reducing the thickness, possibly due to the lateral compressive stress at the Bi/Si interface. Below 3 BL, no coherent phonon signal is detected, in agreement with the phase transformation to the black-phosphoruslike structure. Our observations indicate that the vertical vibrations are significantly softened at 3 BL, but become almost as hard as those in the bulk crystal by adding another bilayer.

We present a simple approach enabling analytical treatments of size effects in ferroelectric samples of complicated shapes for the cases where long-range depolarizing effects are not involved. The key element of the approach is the presentation of the energy of the system as the sum of the bulk and effective surface energy (like in the classical nucleation problem), while the latter is expressed as a function of the bulk value of the order parameter. The effective surface energy is calculated in terms of the Kretschmer-Binder framework. The size-driven shift of $T_{\mathrm{C}}$ in the ferroelectric thin films with in-plane polarization and the nanowires with axial polarization is studied using the proposed approach and the results are compared with those exact. In the limit of large extrapolation length, the approach reproduces the exact results (analytical and numerical). For short extrapolation lengths, it can provide a good approximation to the exact results for the case of second-order phase transitions. For ferroelectrics with the first-order phase transition having the maximal correlation length smaller than the extrapolation length (a common situation in perovskites), the approach provides as well an appropriate description of the size effect on the transition temperature. The proposed approach can be used for the description of the size effect not only in ferroelectrics, but in other ferroics as well.

We formulate a scattering theory to study magnetic films in microwave cavities beyond the independent-spin and rotating-wave approximations of the Tavis-Cummings model. We demonstrate that strong coupling can be realized not only for the ferromagnetic resonance mode, but also for spin-wave resonances; the coupling strengths are mode dependent and decrease with increasing mode index. The strong-coupling regime can also be accessed electrically by spin pumping into a metal contact.

We investigate the limits of effectiveness of classical spin simulations for predicting free induction decays (FIDs) measured by solid-state nuclear magnetic resonance on systems of quantum nuclear spins. The specific limits considered are associated with the range of interaction, the size of individual quantum spins, and the long-time behavior of the FID signals. We compare FIDs measured or computed for lattices of quantum spins (mainly spins 1/2) with the FIDs computed for the corresponding lattices of classical spins. Several cases of excellent quantitative agreement between quantum and classical FIDs are reported along with the cases of gradually decreasing quality of the agreement. We formulate semiempirical criteria defining the situations, when classical simulations are expected to accurately reproduce quantum FIDs. Our findings indicate that classical simulations may be a quantitatively accurate tool of first-principles calculations for a broad class of macroscopic systems, where individual quantum microscopic degrees of freedom are far from the classical limit.

We study the spin-transfer-induced dynamics of a target skyrmion in the free layer of nanopillar structures, in which the polarizer layer has a perpendicular magnetization. By using micromagnetic simulations, it is shown that three distinct modes can be excited. As a consequence of the evolution of the lowest frequency mode, breathing mode, the polarized direction of the skyrmion core can be switched via Bloch point injection and propagation.

We study the mechanism of decay of a topological (winding-number) excitation due to finite-size effects in a two-dimensional valence-bond solid state, realized in an $S=1/2$ spin model $(J-Q$ model) with six-spin interactions and studied using projector Monte Carlo simulations in the valence bond basis. A topological excitation with winding number $\left|W\right|>0$ contains domain walls, which are unstable due to the emergence of long valence bonds in the wave function, unlike in effective descriptions with the quantum dimer model (which by construction includes only short bonds). We find that the lifetime of the winding number in imaginary time, which is directly accessible in the simulations, diverges as a power of the system length $L$. The energy can be computed within this time (i.e., it converges toward a “quasieigenvalue” before the winding number decays) and agrees for large $L$ with the domain-wall energy computed in an open lattice with boundary modifications enforcing a domain wall. Constructing a simplified two-state model which can be solved in real and imaginary time, and using the imaginary-time behavior from the simulations as input, we find that the real-time decay rate out of the initial winding sector is exponentially small in $L$. Thus, the winding number rapidly becomes a well-defined conserved quantum number for large systems, supporting the conclusions reached by computing the energy quasieigenvalues. Including Heisenberg exchange interactions which bring the system to a quantum-critical point separating the valence-bond solid from an antiferromagnetic ground state (the putative “deconfined” quantum-critical point), we can also converge the domain-wall energy here and find that it decays as a power law of the system size. Thus, the winding number is an emergent quantum number also at the critical point, with all winding number sectors becoming degenerate in the thermodynamic limit. This supports the description of the critical point in terms of a U(1) gauge-field theory.

${\mathrm{CeAu}}_2{\mathrm{Si}}_2$ is a newly discovered pressure-induced heavy fermion superconductor, which shows very unusual interplay between superconductivity and magnetism under pressure. Here we compare the results of high-pressure measurements on single-crystalline ${\mathrm{CeAu}}_2{\mathrm{Si}}_2$ samples with different levels of disorder. It is found that while the magnetic properties are essentially sample independent, superconductivity is rapidly suppressed when the residual resistivity of the sample increases. We show that the depression of bulk $T_{\mathrm{c}}$ can be well understood in terms of pair breaking by nonmagnetic disorder, which strongly suggests an unconventional pairing state in pressurized ${\mathrm{CeAu}}_2{\mathrm{Si}}_2$. Furthermore, increasing the level of disorder leads to the emergence of another phase transition at $T^{*}$ within the magnetic phase, which might be in competition with superconductivity.