The influence of nanoscale cavities on the fracture of the $\mathrm{\Sigma }33\left\{554\right\}\left[110\right]$ symmetrical tilt grain boundary is studied by atomistic simulations. The crack crystallography is chosen such that dislocation emission is easy. A transition from a ductile behavior of the tip to a brittle one is obtained for a dense (coverage beyond 15% and intercavity spacing smaller than 4 nm) distribution of small cavities (sizes in-between 1 and 2 nm). The results are in good agreement with recent experiments from the literature. Even at the highest coverage, the character of the crack is highly sensitive to the initial position of the tip and a mixture of ductile and brittle responses is found. This complexity is beyond the usual criterion based on the drop of the work of separation with the amount of damage in the structure. It is shown that a heterogeneous cohesive zone model, with parameters extracted from the simulations and enriched with a criterion for plasticity, can explain the simulations and reproduce the transition. Additional simulations show that outside this range of small sizes and dense packing, which gives essentially a two-dimensional response (either crack opening or infinite straight dislocation emission), dislocation half-loops appear for intercavity spacing starting at about 4 nm. They constitute, together with regions of low coverage/small cavities, efficient obstacles to brittle cracking. These results could be guidelines to designing interfaces more resistant to solute embrittlement, in general. The cohesive zone model is generic. Furthermore, the {554} single crystal was used to determine to which extent the results depend on the details of the core structure versus the cavity distribution. These elements show that the conclusions reached have a generic character.

A model in which a three-dimensional elastic medium is represented by a network of identical masses connected by springs of random strengths and allowed to vibrate only along a selected axis of the reference frame exhibits an Anderson localization transition. To study this transition, we assume that the dynamical matrix of the network is given by a product of a sparse random matrix with real, independent, Gaussian-distributed nonzero entries and its transpose. A finite-time scaling analysis of the system's response to an initial excitation allows us to estimate the critical parameters of the localization transition. The critical exponent is found to be $\nu =1.57\pm 0.02$, in agreement with previous studies of the Anderson transition belonging to the three-dimensional orthogonal universality class.

We study the phase transition between conducting and insulating states taking place in disordered multichannel Luttinger liquids with interchannel interactions. We derive renormalization-group equations which are perturbative in disorder but nonperturbative in interaction. In the vicinity of the simultaneous phase transition in all channels, these equations become a set of coupled Berezinskii-Kosterlitz-Thouless equations, which we analyze within two models: an array of identical wires and a two-channel model with distinct channels. We show that a competition between disorder and interaction results in a variety of phases, expected to be observable at intermediate temperatures where the interaction and disorder are relevant but weak hybridization and the charge-density-wave interaction may be ignored.

Copper borate ${\mathrm{Cu}}_3{\left({\mathrm{BO}}_3\right)}_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper ${\mathrm{Cu}}^{2+}$ ions occupy 16 nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of $3d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space-group symmetry and a large unit cell with $Z=10$. An unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 ${\mathrm{cm}}^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N=10\mathrm{K}$, thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim 1.8\mathrm{eV}$ is observed, which we assign to overlapping of transitions between the $3d^9$ states of ${\mathrm{Cu}}^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim 2.8-3.0$ eV.

We introduce the quantum dimer-pentamer model (QDPM) on the square lattice. This model is a generalization of the square-lattice quantum dimer model as its configuration space comprises fully packed hard-core dimer coverings as well as dimer configurations containing pentamers, where four dimers touch a vertex. Thus in the QDPM, the fully packed, hard-core constraint of the quantum dimer model is relaxed such that the local dimer number at each vertex is fixed modulo 3, resulting in an exact local $Z_3$ gauge symmetry. We construct a local Hamiltonian for which the Rokhsar-Kivelson (RK) equal superposition state is the exact ground state and has a ninefold topological degeneracy on the torus. Using Monte Carlo calculations, we find no spontaneous symmetry breaking in the RK wave function and that its dimer-dimer correlation function decays exponentially. By doping the QDPM RK state with a pair of monomers, we demonstrate that $Z_3$ electric charges are deconfined. Additionally, we introduce a $Z_3$ magnetic string operator that we find decays exponentially and shows no signatures of magnetic vortex condensation. These numerical results suggest that the ground state of the QDPM is a dimer liquid with $Z_3$ topological order.

We prove that Nagaoka's theorem, that the ground state of the large-$U$ Hubbard model with exactly one hole is ferromagnetic, holds for any balanced Hamiltonian. We argue that, in itinerant electron systems, a balanced Hamiltonian, rather than bipartite lattice, defines an unfrustrated system. The proof is valid for multiorbital models with arbitrary two-orbital interactions provided that no exchange interactions are antiferromagnetic: a class of models including the Kanomori Hamiltonian.

Highly sensitive nuclear spin detection is crucial in many scientific areas including nuclear magnetic resonance spectroscopy, magnetic resonance imaging (MRI), and quantum computing. The tiny thermal nuclear spin polarization represents a major obstacle towards this goal which may be overcome by dynamic nuclear spin polarization (DNP) methods. The latter often rely on the transfer of the thermally polarized electron spins to nearby nuclear spins, which is limited by the Boltzmann distribution of the former. Here we utilize microwave dressed states to transfer the high ($>92\%$) nonequilibrium electron spin polarization of a single nitrogen-vacancy center (NV) induced by short laser pulses to the surrounding ${}^{13}\mathrm{C}$ carbon nuclear spins. The NV is repeatedly repolarized optically, thus providing an effectively infinite polarization reservoir. A saturation of the polarization of the nearby nuclear spins is achieved, which is confirmed by the decay of the polarization transfer signal and shows an excellent agreement with theoretical simulations. Hereby we introduce the polarization readout by polarization inversion method as a quantitative magnetization measure of the nuclear spin bath, which allows us to observe by ensemble averaging macroscopically hidden polarization dynamics like Landau-Zener-Stückelberg oscillations. Moreover, we show that using the integrated solid effect both for single- and double-quantum transitions nuclear spin polarization can be achieved even when the static magnetic field is not aligned along the NV's crystal axis. This opens a path for the application of our DNP technique to spins in and outside of nanodiamonds, enabling their application as MRI tracers. Furthermore, the methods reported here can be applied to other solid state systems where a central electron spin is coupled to a nuclear spin bath, e.g., phosphor donors in silicon and color centers in silicon carbide.

Dipole interaction between spins induces a nonzero Berry curvature for magnons in ferromagnetic films, and this Berry curvature gives rise to various phenomena such as thermal Hall effect. In previous works, it has been calculated for magnons with dipole interaction only. Nevertheless, magnon dispersion is largely affected by the exchange interaction neglected in the previous works. In this paper, we calculated Berry curvature of magnons in ferromagnetic films with the dipole and exchange interactions. The resulting Berry curvature shows a sign change by varying the wave number, unlike that with the dipole interaction only. Besides, we calculate the Berry curvature at the anticrossings between magnon modes, and we find that hybridization between eigenmodes at anticrossings gives rise to a large peak in the Berry curvature. In addition, we calculate a shift of the magnon wave packet at the edge of the film coming from the Berry curvature, and it is enhanced at reflection at the anticrossing.

We describe the dynamics of an antiferromagnetic nano-oscillator in an external magnetic field of any given time distribution. The oscillator is powered by a spin current originating from spin-orbit effects in a neighboring heavy metal layer and is capable of emitting a THz signal in the presence of an additional easy-plane anisotropy. We derive an analytical formula describing the interaction between such a system and an external field, which can affect the output signal character. Interactions with magnetic pulses of different shapes, with a sinusoidal magnetic field and with a sequence of rapidly changing magnetic fields are discussed. We also perform numerical simulations based on the Landau-Lifshitz-Gilbert equation with spin-transfer torque effects to verify the obtained results and find a very good quantitative agreement between analytical and numerical predictions.

Ultrafast laser excitation of a metal causes correlated, highly nonequilibrium dynamics of electronic and ionic degrees of freedom, which are, however, only poorly captured by the widely used two-temperature model. Here we develop an out-of-equilibrium theory that captures the full dynamic evolution of the electronic and phononic populations and provides a microscopic description of the transfer of energy delivered optically into electrons to the lattice. All essential nonequilibrium energy processes, such as electron-phonon and phonon-phonon interactions are taken into account. Moreover, as all required quantities are obtained from first-principles calculations, the model gives a realistic and material-dependent description of the relaxation dynamics without the need for fitted parameters. We apply the model to FePt and show that the detailed relaxation is out-of-equilibrium for ps.

An analytical study of the spectral line shape of ferromagnetic resonance (FMR) detected by spin rectification effect and driven by the combined action of spin-transfer torque (STT) and voltage-controlled magnetic anisotropy (VCMA) is developed. The system under consideration consists of a magnetic tunnel junction (MTJ). Explicit expressions for the symmetric and asymmetric components of the rectified voltage are derived, where the role of the VCMA, in-plane STT, and field-like torque is clearly identified and discussed. Typical geometrical configurations are particularly analyzed and compared with recent experimental results. The analytical findings show that the change of sign in the FMR response upon reversal of the magnetization is completely due to VCMA. By distinguishing in-plane, out-of-plane, and full magnetization reversal processes, it is shown that the VCMA induces a change of sign in the symmetric part for the in-plane and out-of-plane magnetization reversal, while the asymmetric part change its sign under a full and in-plane reversion of the magnetization. Explicit expressions of the symmetric and asymmetric contributions of the spectral line shape allow us to detect under what conditions the STT and VCMA can increase or decrease the FMR spectral line shape. The proposed theory allows access to a better understanding of the physics behind ferromagnetic resonance phenomena, promoting potential applications in STT+VCMA-based MTJs.

We report experimental observation of two distinct quantum interference patterns in the absorption spectra when a transmon superconducting qubit is subjected to a bichromatic microwave field with the same Rabi frequencies. Within the two-mode Floquet formalism with no dissipation processes, we propose a graph-theoretical representation to model the interaction Hamiltonian for each of these observations. This theoretical framework provides a clear visual representation of various underlying physical processes in a systematic way beyond rotating-wave approximation. The presented approach is valuable to gain insights into the behavior of multichromatic field driven quantum two-level systems, such as two-level atoms and superconducting qubits. Each of the observed interference patterns is represented by appropriate graph products on the proposed color-weighted graphs. The underlying mechanisms and the characteristic features of the observed fine structures are identified by the transitions between the graph vertices, which represent the doubly dressed states of the system. The good agreement between the numerical simulation and experimental data confirms the validity of the theoretical method. Such multiphoton interference may be used in manipulating the quantum states and/or generate nonclassical microwave photons.

We studied the dynamic instability of the vortex lattice in Nb/permalloy $(\mathrm{permalloy}={\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2})$ bilayers with different Py thickness $d_{\mathrm{Py}}$. When $d_{\mathrm{Py}}$ is larger than some critical value, $d_{\mathrm{cr}}^{\mathrm{ESD}}$, which in our samples has been evaluated to be $\simeq 125$ nm, an inhomogeneous magnetic configuration is induced in the samples as a precursor to a fully developed stripe domain state. We found that this inhomogeneous magnetic configuration of the Py layer promotes larger values of the critical velocity for the vortex lattice instability, $v^{*}$, with respect to the homogeneous case.

We present the design of an inductively shunted transmon qubit with flux-tunable coupling to an embedded harmonic mode. This circuit construction offers the possibility to flux-choose between pure transverse and pure longitudinal coupling, that is, coupling to the ${\sigma }_x$ or ${\sigma }_z$ degree of freedom of the qubit. While transverse coupling is the coupling type that is most commonly used for superconducting qubits, the inherently different longitudinal coupling has some remarkable advantages both for readout and for the scalability of a circuit. Being able to choose between both kinds of coupling in the same circuit provides the flexibility to use one for coupling to the next qubit and one for readout, or vice versa. We provide a detailed analysis of the system's behavior using realistic parameters, along with a proposal for the physical implementation of a prototype device.

Finding concrete realizations for topologically nontrivial chiral superconductivity has been a long-standing goal in quantum matter research. Here, we propose a route to a systematic realization of chiral superconductivity with nonzero Chern numbers. This goal can be achieved in a nanomagnet lattice deposited on top of a spin-orbit coupled two-dimensional electron gas (2DEG) with proximity $s$-wave superconductivity. The proposed structure can be regarded as a universal platform for chiral superconductivity supporting a large variety of topological phases. The topological state of the system can be electrically controlled by, for example, tuning the density of the 2DEG.

In this paper, the effects of cobalt substitution on the transport and electronic properties of the recently discovered iron-based superconductor $\mathrm{KC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$, with $T_{\mathrm{c}}=33\mathrm{K}$, are reported. This material is an unusual superconductor showing intrinsic hole conduction ($0.25\mathrm{holes}/\mathrm{F}{\mathrm{e}}^{2+}$). Upon doping of Co, the $T_{\mathrm{c}}$ of $\mathrm{KC}{\mathrm{a}}_2{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$ gradually decreased, and bulk superconductivity disappeared when $x\ge 0.25$. Conversion of the primary carrier from $p$ type to $n$ type upon Co-doping was clearly confirmed by Hall measurements, and our results are consistent with the change in the calculated Fermi surface. Nevertheless, neither spin density wave (SDW) nor an orthorhombic phase, which are commonly observed for nondoped iron-based superconductors, was observed in the nondoped or electron-doped samples. The electron count in the $3d$ orbitals and structural parameters were compared with those of other iron-based superconductors to show that the physical properties can be primarily ascribed to the effects of disorder.

We investigate and map out the nonequilibrium phase diagram of a generalization of the well known Aubry-André-Harper (AAH) model. This generalized AAH (GAAH) model is known to have a single-particle mobility edge which also has an additional self-dual property akin to that of the critical point of the AAH model. By calculating the population imbalance, we get hints of a rich phase diagram. We also find a fascinating connection between single particle wave functions near the mobility edge of the GAAH model and the wave functions of the critical AAH model. By placing this model far from equilibrium with the aid of two baths, we investigate the open system transport via system size scaling of nonequilibrium steady state (NESS) current, calculated by fully exact nonequilibrium Green's function (NEGF) formalism. The critical point of the AAH model now generalizes to a ‘critical’ line separating regions of ballistic and localized transport. Like the critical point of the AAH model, current scales subdiffusively with system size on the ‘critical’ line ($I\sim N^{-2\pm 0.1}$). However, remarkably, the scaling exponent on this line is distinctly different from that obtained for the critical AAH model (where $I\sim N^{-1.4\pm 0.05}$). All these results can be understood from the above-mentioned connection between states near the mobility edge of the GAAH model and those of the critical AAH model. A very interesting high temperature nonequilibrium phase diagram of the GAAH model emerges from our calculations.