We propose to apply spin noise spectroscopy (SNS) to detect many-body localization (MBL) in disordered spin systems. The SNS methods are relatively noninvasive techniques to probe spontaneous spin fluctuations. Here, we show that the spin noise signals obtained by cross-correlation SNS with two probe beams can be used to separate the MBL phase from a noninteracting Anderson localized phase and a delocalized (diffusive) phase in the studied models for which we numerically calculate real-time spin noise signals and their power spectra. For an archetypical case of the disordered XXZ spin chain, we also develop a simple phenomenological model.

Molecular-dynamics simulations combined with a two-temperature model are used to study laser ablation in ${\text{Cu}}_x{\text{Zr}}_{1-x}$ $\left(x=0.33,0.50,0.67\right)$ metallic glasses as well as crystalline ${\text{CuZr}}_2$ in the ${\text{C11}}_b$ $\left({\text{MoSi}}_2\right)$ structure. Ablation thresholds are found to be $430\pm 10,450\pm 10,510\pm 10$, and $470\pm 10$ ${\mathrm{J}/\mathrm{m}}_2$ for a-${\text{Cu}}_2\text{Zr}$, a-CuZr, ${\text{a-CuZr}}_2$, and ${\text{c-CuZr}}_2$, respectively. The larger threshold in amorphous ${\text{CuZr}}_2$ results from a weaker electron-phonon coupling and thus longer electron-ion equilibration time. We observe that the velocity of the pressure waves in the amorphous samples is not affected by the fluence, in contrast to the crystal; this is due to differences in the behavior of the shear modulus with increasing pressure. The heat-affected zone in the different systems is characterized in terms of the melting depth as well as inelastic deformations. The melting depth is found to be smaller in the crystal than in the amorphous targets because of its higher melting temperature. The inelastic deformations are investigated in terms of the von Mises shear strain invariant ${\eta }^{\text{Mises}}$; the homogeneous nucleation of shear transformation zones is observed in the glass as reported in previous theoretical and experimental studies. The coalescence of the shear transformation zones is also found at higher fluence.

This paper introduces a phase-field crystal (PFC) approach that couples the atomic-scale PFC density field to order parameters describing ferromagnetic and ferroelectric ordering, as well to a solute impurity field. This model extends the magnetic PFC model introduced by Faghihi et al. [N. Faghihi, Ph.D. Thesis, The University of Western Ontario, 2012; N. Faghihi, N. Provatas, K. R. Elder, M. Grant, and M. Karttunen, Phys. Rev. E 88, 032407 (2013)] to incorporate polarization and concentration fields, as well as anisotropic ordering of the magnetization and polarization fields as determined by the local crystalline orientation. Magnetoelectric coupling is incorporated through the elastic coupling. Analytic calculations for a body centered-cubic (BCC) system are presented to illustrate that the model reduces to the standard multiferroic phase-field models when only a single crystal is considered. Two special cases of the model are then studied, the first focusing on magnetocrystalline interactions in a system described by the two-point correlation function of the XPFC model developed by Greenwood et al. [M. Greenwood, N. Provatas, and J. Rottler, Phys. Rev. Lett. 105, 045702 (2010); M. Greenwood, J. Rottler, and N. Provatas, Phys. Rev. E 83, 031601 (2011)], and the second focusing on electrocrystalline interactions in a system described by the original PFC kernel developed by Elder et al. K. R. Elder, M. Katakowski, M. Haataja, and M. Grant, Phys. Rev. Lett. 88, 245701 (2002); K. R. Elder and M. Grant, Phys. Rev. E 70, 051605 (2004)]. We examine the small deformation properties of these two realizations of the model . Numerical simulations are performed to illustrate how magnetocrystalline coupling can be exploited to design a preferential grain texture and how defects and grain boundaries influence the ferroelectric coercivity.

Ultrafast studies of magnetization dynamics have revealed fundamental processes that govern spin dynamics, and the emergence of time-resolved x-ray techniques has extended these studies to long-range spin structures that result from interactions with competing symmetries. By combining time-resolved resonant x-ray scattering and ultrafast magneto-optical Kerr studies, we show that the dynamics of the core spins in the helical magnetic structure occur on much longer time scales than the excitation of conduction electrons in the lanthanide metal Dy. The observed spin behavior differs markedly from that observed in the ferromagnetic phase of other lanthanide metals or transition metals and is strongly dependent on temperature and excitation fluence. This unique behavior results from coupling of the real-space helical spin structure to the shape of the conduction electron Fermi surface in momentum space, which creates a bottleneck in spin scattering events that transfer the valence excitation to the core spins. The dependence of the dynamics on the intersite interactions renders the helical ordering much more robust to perturbations than simple ferromagnetic or antiferromagnetic ordering, where dynamics are driven primarily by on-site interactions.

We investigate the electronic and magnetic properties of a two-dimensional polymeric manganese phthalocyanine (Mn-Pc) network and its derivative, the metal-ligand network Mn-TCNB formed by the transition-metal atom Mn and the organic ligand TCNB (1,2,4,5-tetracyanobenzene), using first-principles calculations on the basis of density functional theory (DFT) with the Hubbard-like Coulomb term. Our calculations show that Mn-Pc and Mn-TCNB are metallic. It is found that the Mn-Pc network is more stable than the Mn-TCNB one, and both have a total magnetic moment of about $3{\mu }_B$. In the case of Mn-Pc, also the local Mn moment is close to $3{\mu }_B$. But in Mn-TCNB, we find a high spin state $S=5/2$ at Mn that is partially screened by unpaired electrons at the ligands. That screening is static in the $\mathrm{DFT}+U$ results, but we argue in favor of a dynamical screening in reality. Using our proper model calculation on the basis of a suitable model Hamiltonian, we explain the ab initio calculations, analyze the partial screening effect that exists in the two-dimensional Mn-TCNB network, and compare both systems.

Observing constituent particles with fractional quantum numbers in confined and deconfined states is an interesting and challenging problem in quantum many-body physics. Here, we further explore a computational scheme [Y. Tang and A. W. Sandvik, Phys. Rev. Lett. 107, 157201 (2011)] based on valence-bond quantum Monte Carlo simulations of quantum spin systems. Using several different one-dimensional models, we characterize $S=\frac{1}{2}$ spinon excitations using the intrinsic spinon size $\lambda$ and confinement length $\mathrm{\Lambda }$ (the size of a bound state). The spinons have finite size in valence-bond-solid states, infinite size in the critical region (with overlaps characterized by power laws), and become ill defined (completely unlocalizable) in the Néel state (which we stabilize in one dimension by introducing long-range interactions). We also verify that pairs of spinons are deconfined in uniform spin chains but become confined upon introducing a pattern of alternating coupling strengths (dimerization) or coupling two chains (forming a ladder). In the dimerized system, an individual spinon can be small when the confinement length is large; this is the case when the imposed dimerization is weak but the ground state of the corresponding uniform chain is a spontaneously formed valence-bond solid (where the spinons are deconfined). Based on our numerical results, we argue that a system with $\lambda \ll \mathrm{\Lambda }$ is associated with weak repulsive short-range spinon-spinon interactions. In principle, both the length scales $\lambda$ and $\mathrm{\Lambda }$ can still be individually tuned from small to infinite (with $\lambda \le \mathrm{\Lambda }$) by varying model parameters. In contrast, in the ladder system the two lengths are always similar, and this is the case also in the weakly dimerized systems when the corresponding uniform chain is in the critical phase. In these systems, the effective spinon-spinon interactions are purely attractive and there is only a single large length scale close to criticality, which is reflected in the standard spin correlations as well as in the spinon characteristics.

Topological quantum phases of matter have been a topic of intense interest in contemporary condensed matter physics. Extensive efforts are devoted to investigate various exotic properties of topological matter including topological insulators, topological superconductors, and topological semimetals. For topological insulators, the dissipationless transport via gapless helical edge or surface states is supposed to play a defining role, which unfortunately has proved difficult to realize in experiments due to inevitable backscattering induced in the sample boundary. Motivated by the fundamental connection between topological invariants and the Zak phase, here, we show that the nontrivial band topologies of both two- and three-dimensional topological insulators, characterized by the Chern numbers and the ${\mathbb{Z}}_2$ invariants, respectively, are directly manifested in the winding numbers of the Wannier-Stark ladder (WSL) emerging under an electric field. We use the Floquet Green's function formalism to show that the winding number of the WSL is robust against interband interference as well as nonmagnetic impurity scattering.

We test three different approaches, based on quantum Monte Carlo simulations, for computing the velocity $c$ of triplet excitations in antiferromagnets. We consider the standard $S=1/2$ one- and two-dimensional Heisenberg models, as well as a bilayer Heisenberg model at its critical point. Computing correlation functions in imaginary time and using their long-time behavior, we extract the lowest excitation energy versus momentum using improved fitting procedures and a generalized moment method. The velocity is then obtained from the dispersion relation. We also exploit winding numbers to define a cubic space-time geometry, where the velocity is obtained as the ratio of the spatial and temporal lengths of the system when all winding number fluctuations are equal. The two methods give consistent results for both ordered and critical systems, but the winding number estimator is more precise. For the Heisenberg chain, we accurately reproduce the exactly known velocity. For the two-dimensional Heisenberg model, our results are consistent with other recent calculations, but with an improved statistical precision, $c=1.65847\left(4\right)$. We also use the hydrodynamic relation $c^2={\rho }_s/{\chi }_{\perp }(q\to 0)$ between $c$, the spin stiffness ${\rho }_s$, and the transversal susceptibility ${\chi }_{\perp }$, using the smallest non-zero momentum $q=2\pi /L$. This method also is well controlled in two dimensions, but the cubic criterion for winding numbers delivers better numerical precision. In one dimension, the hydrodynamic relation is affected by logarithmic corrections which make accurate extrapolations difficult. As an application of the winding number method, for the quantum-critical bilayer model our high-precision determination of the velocity enables us to quantitatively test, at an unprecedented level, the field-theoretic low-temperature scaling forms $\chi =(a/c^2)T$ and $C=(b/c^2)T^2$. We find agreement to within $3\%$ with the leading $1/N$ results for the factors $a$ and $b$ in the $\mathrm{O}\left(N\right)$ model, but the agreement becomes significantly worse when the first corrections in $1/N$ are considered.

We study one-dimensional quantum systems near the classical limit described by the Korteweg–de Vries (KdV) equation. The excitations near this limit are the well-known solitons and phonons. The classical description breaks down at long wavelengths, where quantum effects become dominant. Focusing on the spectra of the elementary excitations, we describe analytically the entire classical-to-quantum crossover. We show that the ultimate quantum fate of the classical KdV excitations is to become fermionic particles and holes. We discuss in detail two exactly solvable models exhibiting such crossover, namely the Lieb-Liniger model of bosons with weak contact repulsion and the quantum Toda model. We argue that the results obtained for these models are universally applicable to all quantum one-dimensional systems with a well-defined classical limit described by the KdV equation.

In strong topological insulators protected surface states are always manifest, while in weak topological insulators (WTIs) the corresponding metallic surface states are either manifest or hidden, depending on the orientation of the surface. One can design a nanostep on the surface of a WTI such that a protected helical channel appears along it. In a more generic WTI nanostructure, multiple sets of such quasi-one-dimensional (1D) channels emerge and are coupled to each other. We study the response of the electronic spectrum associated with such quasi-1D surface modes against a magnetic flux piercing the system in the presence of disorder, and we find a nontrivial, connected spectral flow as a clear signature indicating the immunity of the surface modes to disorder. We propose that the WTI nanoarchitecture is a promising platform for realizing topologically protected nanocircuits immune to disorder.

We perform a theoretical investigation of light propagation and Anderson localization in one-dimensional disordered superlattices composed of dielectric stacks with graphene sheets in between. Disorder is introduced either on graphene material parameters (e.g., Fermi energy) or on the widths of the dielectric stacks. We derive an analytic expression for the localization length $\xi$, and we compare it to numerical simulations using the transfer-matrix technique; a very good agreement is found. We demonstrate that the presence of graphene may strongly attenuate the anomalously delocalized Brewster modes, and it is at the origin of a periodic dependence of $\xi$ on frequency, in contrast to the usual asymptotic decay, $\xi \propto {\omega }^{-2}$. By unveiling the effects of graphene on Anderson localization of light, we pave the way for new applications of graphene-based, disordered photonic devices in the THz spectral range.

We show theoretically that in an external magnetic field, like charges on top of graphene monolayer may be mutually attracted to form macromolecules. For this to happen, graphene needs to be in a quantum Hall plateau state with the local chemical potential being between the Landau levels. One or several graphene electron(s) get localized in the middle between charges and provide overscreening of the Coulomb repulsion between the charges. The size of the resulting macromolecules is of the order of the magnetic length ($\sim 10$ nm for magnetic field 10 T). The possible stable macromolecules that unit charges can form on graphene in a magnetic field are classified. The binding survives significant temperatures, exceeding the mobility barriers for many ionically bond impurities. The influence of possible lattice-scale effects of valley mixing are discussed. Tuning the doping of graphene or the magnetic field, the binding of impurities can be turned on and off and the macromolecule size may be tuned. This opens the perspective to nanoscopic manipulation of ions on graphene by using magnetic field and gating.

The surface termination of ${\mathrm{CePt}}_5/\mathrm{Pt}\left(111\right)$ is determined experimentally by LEED-IV. In accordance with recent theoretical predictions, a dense Pt terminated surface is being found. Whereas the ${\mathrm{CePt}}_5$ volume lattice comprises Pt kagome layers, additional Pt atoms occupy the associated hole positions at the surface. This finding provides a natural explanation for the remarkable inertness of the ${\mathrm{CePt}}_5$ intermetallic. Implications of the structural relaxations determined by LEED-IV analysis are discussed with regard to observations by scanning tunneling microscopy and electron spectroscopies.

We develop tangent space methods for projected entangled-pair states (PEPS) that provide direct access to the low-energy sector of strongly-correlated two-dimensional quantum systems. More specifically, we construct a variational ansatz for elementary excitations on top of PEPS ground states that allows for computing gaps, dispersion relations, and spectral weights directly in the thermodynamic limit. Solving the corresponding variational problem requires the evaluation of momentum transformed two-point and three-point correlation functions on a PEPS background, which we can compute efficiently by using a contraction scheme. As an application we study the spectral properties of the magnons of the Affleck-Kennedy-Lieb-Tasaki model on the square lattice and the anyonic excitations in a perturbed version of Kitaev's toric code.

We use single-crystal neutron diffraction to determine the crystal structure symmetry and the magnetic evolution in the rhodium-doped iridates ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_4$ ($0\le x\le 0.16$). Throughout this doping range, the crystal structure retains a tetragonal symmetry (space group $I{4}_1/a$) with two distinct magnetic Ir sites in the unit cell forming staggered ${\mathrm{IrO}}_6$ rotation. Upon Rh doping, the magnetic order is suppressed and the magnetic moment of ${\mathrm{Ir}}^{4+}$ is reduced from $0.21{\mu }_{\mathrm{B}}$/Ir for $x=0$ to $0.18{\mu }_{\mathrm{B}}$/Ir for $x=0.12$. The magnetic structure at $x=0.12$ is different from that of the parent compound, while the moments remain in the basal plane.

We study the $\mathrm{C}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{e}}_x$ phase diagram finding that its phase transition temperature towards an antiferromagnetic order $T_N$ follows a quantum ${\left[\left(x_c-x\right)/x_c\right]}^{\psi }$ law, with $\psi =1/2$, from the quantum critical point (QCP) at $x_c=0.25$ up to $T_N\approx 600\mathrm{K}$. We compare this system to others in order to understand why this elemental material is affected by the QCP up to such unusually high temperatures. We determine a general criterion for the crossover, as a function of an external parameter such as concentration, from the region controlled solely by thermal fluctuations to that where quantum effects become observable. The properties of materials with low coherence lengths will thus be altered far away from the QCP.

The electronic and magnetic properties of cation vacancies in $m\text{−}{\mathrm{HfO}}_2$ are predicted using density functional theory. The hafnium vacancy is found to introduce a series of charge transition levels in the range 0.76–1.67 eV above the valence band maximum associated with holes localized on neighboring oxygen sites. The neutral defect adopts a $S=2$ spin state, and we compute corresponding $g$ tensors to aid electron experimental identification of the defect by electron spin resonance spectroscopy. We find that separated vacancies exhibit weak ferromagnetic coupling and the interaction is highly anisotropic—being much stronger when mediated by planes of three-coordinated oxygen ions. Further, we characterize the process of thermal detachment of a hole from a neutral vacancy providing an atomistic model for the $p$-type conductivity observed experimentally at high temperature. These results provide invaluable information on the electronic and magnetic properties of cation vacancies in ${\mathrm{HfO}}_2$ and can aid future experimental identification of these complex defects.