Magnetic systems have been extensively studied from both a fundamental physics perspective and as technological building blocks. The topological properties of magnonic excitations in these systems remain relatively unexplored, due to their inherently dissipative nature. The recent extension of the theory of topological classification to non-Hermitian Hamiltonians provides a pathway to engineer topological phases in dissipative systems. Here, we propose a magnonic realization of a topological, non-Hermitian system. A crucial ingredient of our proposal is the injection of spin current into the magnetic system, which alters and can even change the sign of terms describing dissipation. We show that the magnetic dynamics of an array of spin-torque oscillators can be mapped onto a non-Hermitian Su-Schrieffer-Heeger model exhibiting topologically protected edge states. The nontrivial topological phase is accessed by tuning the spin current injected into the array. We derive this result using both exact diagonalization of the effective non-Hermitian Hamiltonian and numerical analysis of the nonlinear equations of motion. In the nontrivial topological phase, a single spin-torque oscillator on the edge of the array is driven into auto-oscillation and emits a microwave signal, while the bulk oscillators remain inactive. Our findings have practical utility for memory devices and spintronics neural networks relying on spin-torque oscillators as constituent units.

The interplay of time-reversal and $n$-fold rotation symmetries ($n=2,4,6$) is known to bring a new class of topological crystalline insulators (TCIs) having $n$ surface Dirac cones due to a surface rotation anomaly. We show that the proximity-induced $s$-wave superconductivity on the surface of these TCIs yields a topological superconducting phase in which two Majorana zero modes are bound to a vortex, and that $n$-fold rotation symmetry ($n=2,4,6$) enriches the topological classification of a superconducting vortex from ${\mathbb{Z}}_2$ to ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$. Using a model of a three-dimensional high-spin topological insulator with $s$-wave superconductivity and twofold rotation symmetry, we show that, with increasing chemical potential, the number of Majorana zero modes at one end of a vortex changes as $2\to 1\to 0$ through two topological vortex phase transitions. In addition, we show that additional magnetic-mirror symmetry further enhances the topological classification to $\mathbb{Z}\times \mathbb{Z}$.

The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics simulations coupled to molecular dynamics, using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom $M\left(T\right)$, including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on $M\left(T\right)$ are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering. Analytical approximations to the magnetization as a function of temperature and size are proposed.

$\alpha -\mathrm{Co}{\mathrm{V}}_2{\mathrm{O}}_6$ consists of $j_{\mathrm{eff}}=\frac{1}{2}$ Ising spins located on an anisotropic triangular motif with magnetization plateaus in an applied field. We combine neutron diffraction with low temperature magnetization to investigate the magnetic periodicity in the vicinity of these plateaus. We find these steps to be characterized by metastable and spatially short-range ($\xi \sim 10$ Å) magnetic correlations with antiphase boundaries defining a local periodicity of $\langle {\widehat{T}}^2\rangle =\uparrow \downarrow$ to $\langle {\widehat{T}}^3\rangle =\uparrow \uparrow \downarrow$ and $\langle {\widehat{T}}^4\rangle =\uparrow \uparrow \downarrow \downarrow$ or $\uparrow \uparrow \uparrow \downarrow$ spin arrangements. This shows the presence of spatially short range and metastable/hysteretic, commensurate magnetism in Ising magnetization steps.

We report the evolution of the magnetic properties of ${\mathrm{Ce}}_2{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_{8-y}{\mathrm{Cd}}_y$ single crystals. In particular, for ${\mathrm{Ce}}_2{\mathrm{Rh}}_{0.5}{\mathrm{Ir}}_{0.5}{\mathrm{In}}_8$ ($T_N$ = 2.0 K) and ${\mathrm{Ce}}_2{\mathrm{Rh}}_{0.5}{\mathrm{Ir}}_{0.5}{\mathrm{In}}_{7.79}{\mathrm{Cd}}_{0.21}$ ($T_N$ = 4.2 K), we have solved the magnetic structure of these compounds using single-crystal neutron magnetic diffraction experiments. Taking the magnetic structure of the ${\mathrm{Ce}}_2\mathrm{Rh}{\mathrm{In}}_8$ heavy-fermion antiferromagnet as a reference, we have identified no changes in the $\mathbf{q}=(\frac{1}{2},\frac{1}{2},0)$ magnetic wave vector; however, the direction of the ordered ${\mathrm{Ce}}^{3+}$ moments rotates toward the $ab$ plane, under the influence of both dopants. By constraining the analysis of the crystalline electric field (CEF) with the experimental ordered moment's direction and high-temperature magnetic-susceptibility data, we have used a mean-field model with tetragonal CEF and exchange interactions to gain insight into the CEF scheme and anisotropy of the CEF ground-state wave function when Cd and Ir are introduced into ${\mathrm{Ce}}_2\mathrm{Rh}{\mathrm{In}}_8$. Consistent with previous work, we find that Cd doping in ${\mathrm{Ce}}_2\mathrm{Rh}{\mathrm{In}}_8$ tends to rotate the magnetic moment toward the $ab$ plane and lower the energy of the CEF excited states' levels. Interestingly, the presence of Ir also rotates the magnetic moment towards the $ab$ plane although its connection to the CEF overall splitting evolution for the $y$ = 0 samples may not be straightforward. These findings may shed light on the origin of the disordered spin-glass phase on the Ir-rich side of the phase diagram and also indicate that the ${\mathrm{Ce}}_{2}M{\mathrm{In}}_8$ compounds may not follow exactly the same Rh-Ir CEF effects trend established for the $\mathrm{Ce}M{\mathrm{In}}_5$ compounds.

We propose a computationally efficient Kerker mixing scheme for robust and rapidly converging self-consistent-field calculations using all-electron first-principles electronic structure methods based on the muffin-tin partitioning of space. The mixing scheme is composed of the Kerker preconditioner in combination with quasi-Newton methods. We construct the Kerker preconditioner in the muffin-tin sphere by determining the screened Coulomb potential in real space, solving a modified Helmholtz equation by adopting Weinert's pseudocharge method for calculating the Poisson equation for periodic charge densities without shape approximation to the solution of the modified Helmholtz equation. Implemented in a full-potential linearized augmented plane-wave (FLAPW) method, we found that the Kerker preconditioning scheme (i) leads to a convergence to self-consistency that is independent of system size, (ii) is extremely robust in the choice of the mixing and preconditioning parameters, (iii) scales linearly with system size in computational cost, and (iv) conserves the total charge. We have related the preconditioning parameter to the density of states of the delocalized electrons at the Fermi energy and developed a model to choose the preconditioning parameter either prior to the calculation or on the fly. Our computationally validated model supports the hypothesis that, in the absence of Kerker preconditioning, the delocalized $s$ and $p$ electrons of simple and transition metals are the primary cause for the slowing of the convergence speed and that the stronger, localized $d$ and $f$ electrons account for only a small fraction of the charge sloshing problem. The presented formulation of the Kerker preconditioning scheme establishes an efficient methodology for the simulation of magnetic and nonmagnetic metallic large-scale material systems by means of muffin-tin-based all-electron methods.

Localized magnons states, due to flat bands in the spectrum, is an intensely studied phenomenon and can be found in many frustrated magnets of different spatial dimensionality. The presence of Dzyaloshinskii-Moriya (DM) interactions may change radically the behavior in such systems. In this context, we study a paradigmatic example of a one-dimensional frustrated antiferromagnet, the sawtooth chain in the presence of DM interactions. Using both path integrals methods and numerical density matrix renormalization group, we revisit the physics of localized magnons and determine the consequences of the DM interaction on the ground state. We have studied the spin current behavior, finding three different regimes. First, a Luttinger-liquid regime where the spin current shows a step behavior as a function of parameter $D$, at a low magnetic field. Increasing the magnetic field, the system is in the Meissner phase at the $m=1/2$ plateau, where the spin current is proportional to the DM parameter. Finally, further increasing the magnetic field and for finite $D$ there is a small stiffness regime where the spin current shows, at fixed magnetization, a jump to large values at $D=0$, a phenomenon also due to the flat band.

We study the energy dispersion relation of the $\pi$ and ${\pi }^{*}$ bands in epitaxial monolayer graphene on a semi-infinite Ir(111) substrate by a first-principles density-functional calculation. For this purpose, we employ a realistic surface structure in which the $(10\times 10)$ unit cell of graphene matches a $(9\times 9)$ cell of Ir(111). We determine the surface geometry by using a slab model containing four Ir layers, and the optimized structure is used as input for the subsequent surface embedded Green's function calculation. By taking advantage of semi-infinite calculations, we discuss mini energy gaps at the crossing of the $\pi$ band and its replicas, the Rashba-type spin splitting of the $\pi$ and ${\pi }^{*}$ bands, and also the energy width of both bands arising from interactions with the energy continuum of bulk Ir bands.

We discuss the nonradiative heat transfer in a nonequilibrium double-layer graphene system. We show that at the neutrality point the heat exchange is dominated by the interlayer plasmon modes and derive analytic expressions for the heat current as a function of temperature and the interlayer separation. These results show that for a range of low temperatures the two graphene layers are a much more efficient heat exchanger than conventional metals. The physical reason behind this phenomenon is the presence of interband excitations with a large energy, and a small momentum in the graphene spectrum. The plasmonic mechanism of the heat transfer is sharply suppressed by electrostatic doping. This allows for tuning of the heat exchange by a small applied voltage between the two layers.

First-quantized deep neural network techniques are developed for analyzing strongly coupled fermionic systems on the lattice. Using a Slater-Jastrow-inspired ansatz which exploits deep residual networks with convolutional residual blocks, we approximately determine the ground state of spinless fermions on a square lattice with nearest-neighbor interactions. The flexibility of the neural-network ansatz results in a high level of accuracy when compared with exact diagonalization results on small systems, both for energy and correlation functions. On large systems, we obtain accurate estimates of the boundaries between metallic and charge-ordered phases as a function of the interaction strength and the particle density.

A many-electron conducting system undergoes free acceleration in response to a macroscopic field. The Drude weight $D$—also called charge stiffness—measures the adiabatic (inverse) inertia of the electrons; the $D$ formal expression requires periodic boundary conditions. When instead a bounded sample is addressed within open boundary conditions, no current flows and a constant (external) field only polarizes the sample: the Faraday cage effect. Nonetheless, a low-frequency field induces forced oscillations: We show here that the low-frequency linear response of the bounded system is dominated by the adiabatic inertia and allows an alternative evaluation of $D$. Simulations on model one-dimensional systems demonstrate our main message.

A topological insulator doped with random magnetic impurities is studied. The system is modeled by the Kane-Mele model with a random spin exchange between conduction electrons and magnetic dopants. The dynamical mean-field theory for disordered systems is used to investigate the electron dynamics. The magnetic long-range order and the topological invariant are calculated within the mean-field theory. They reveal rich phases, where different magnetic long-range orders such as antiferromagnetic or ferromagnetic one can exist in the metallic or insulating phases, depending on electron and magnetic impurity fillings. It is found that insulator only occurs at electron half filling, quarter filling and when electron filling is equal to magnetic impurity filling. However, nontrivial topology is observed only in half-filling antiferromagnetic insulator and quarter-filling ferromagnetic insulator. At electron half filling, the spin Hall conductance is quantized and it is robust against magnetic doping, while at electron quarter filling, magnetic dopants drive the ground state from ferromagnetic topological insulator to ferromagnetic metal. The quantum anomalous Hall effect is observed only at electron quarter filling and dense magnetic doping.

Classification of the nonequilibrium quantum many-body dynamics is a challenging problem in condensed matter physics and statistical mechanics. In this work, we study the basic question that whether a ($1+1$) dimensional conformal field theory (CFT) is stable or not under a periodic driving with $N$ noncommuting Hamiltonians. Previous works showed that a Floquet (or periodically driven) CFT driven by certain ${SL}_2$ deformed Hamiltonians exhibit both nonheating (stable) and heating (unstable) phases. In this work, we show that the phase diagram depends on the types of driving Hamiltonians. In general, the heating phase is generic, but the nonheating phase may be absent in the phase diagram. For the existence of the nonheating phases, we give sufficient and necessary conditions for $N=2$ and sufficient conditions for $N>2$. These conditions are composed of $N$ layers of data, with each layer determined by the types of driving Hamiltonians. Our results also apply to the single quantum quench problem with $N=1$.

We analyze multiorbital Hubbard models describing Hund's metals, focusing on the ubiquitous occurrence of a charge instability, signaled by a divergent/negative electronic compressibility, in a range of doping from the half-filled Mott insulator corresponding to the frontier between Hund's and normal metals. We show that the breaking of rotational invariance favors this instability: both spin anisotropy in the interaction and crystal-field splitting among the orbitals make the instability zone extend to larger dopings, making it relevant for real materials like iron-based superconductors. These observations help us build a coherent picture of the occurrence and extent of this instability. We trace it back to the partial freezing of the local degrees of freedom in the Hund's metal, which reduces the allowed local configurations and thus the quasiparticle itinerancy. The abruptness of the unfreezing happening at the Hund's metal frontier can be directly connected to a rapid change in the electronic kinetic energy and thus to the enhancement and divergence of the compressibility.

Building on large-scale quantum Monte Carlo simulations, we investigate the zero-temperature phase diagram of hard-core bosons in a random potential on site-centered Cayley trees with branching number $K=2$. In order to follow how the Bose-Einstein condensate (BEC) is affected by the disorder, we focus on both the zero-momentum density, probing the quantum coherence, and the one-body density matrix (1BDM) whose largest eigenvalue monitors the off-diagonal long-range order. We further study its associated eigenstate which brings useful information about the real-space properties of this leading eigenmode. Upon increasing randomness, we find that the system undergoes a quantum phase transition at finite disorder strength between a long-range ordered BEC state, fully ergodic at large scale, and a new disordered Bose glass regime showing conventional localization for the coherence fraction while the 1BDM displays a nontrivial algebraic vanishing BEC density together with a nonergodic occupation in real space. These peculiar properties can be analytically captured by a simple phenomenological description on the Cayley tree which provides a physical picture of the Bose glass regime.

We study phases and transitions of the square-lattice double dimer model, consisting of two coupled replicas of the classical dimer model. As on the cubic lattice, we find a thermal phase transition from the Coulomb phase, a disordered but correlated dimer liquid, to a phase where fluctuations of the two replicas are closely synchronized with one another. Surprisingly, and in contrast to the cubic case, the phase boundary includes the noninteracting point, as we establish using a symmetry-based analysis of an effective height theory, indicating that infinitesimal coupling is sufficient to synchronize the double dimer model. In addition, we observe a novel antisynchronized phase when the coupling between replicas is repulsive, and use Monte Carlo simulations to establish the full phase diagram, including (anti)synchronized columnar and staggered phases, with interactions between parallel dimers in each replica.

A theory of superconductivity is presented where the effect of anharmonicity, as entailed in the acoustic, or optical, phonon damping, is explicitly considered in the pairing mechanism. The gap equation is solved including diffusive Akhiezer damping for longitudinal acoustic phonons or Klemens damping for optical phonons, with a damping coefficient which, in either case, can be directly related to the Grüneisen parameter and hence to the anharmonic coefficients in the interatomic potential. The results show that the increase of anharmonicity has a strikingly nonmonotonic effect on the critical temperature $T_c$. The optimal damping coefficient yielding maximum $T_c$ is set by the velocity of the bosonic mediator. This theory may open up unprecedented opportunities for material design where $T_c$ may be tuned via the anharmonicity of the interatomic potential, and presents implications for the superconductivity in the recently discovered hydrides, where anharmonicity is very strong and for which the anharmonic damping is especially relevant.

Lattice dynamics determines a number of important properties of solids. While computational methods with predictive power have been developed in this area, the task is still difficult for the complex compounds. We present a method for automatic on-the-fly generation of multicomponent interatomic potentials. The method is based on active learning, which ensures effective extrapolation to new atomic environments. The accuracy is then demonstrated on the example of the Yb-filled skutterudite compound ${\mathrm{Yb}}_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$, which is a family of the promising thermoelectric materials. Atomic displacements, vibrational spectrum, and lattice thermal conductivity were obtained and the effect of the Yb filling and ordering was studied as 700 K. The potential allowed us to reproduce fine features of the vibrational spectrum, as well as the reduction of the lattice thermal conductivity with filling. We found only a small effect of the disorder on the vibrational spectrum and the thermal conductivity.

We investigate the temperature effects on the spin current through an interface between a normal metal and a quasi-two-dimensional ferromagnetic insulator. Conductive electrons are reflected at the interface absorbing or emitting magnons. The interaction process depends on the temperature, and we are interested in finding out how the transport of spin current is affected close to the Berezinskii-Kosterlitz-Thouless (BKT) transition. That is an important open question. While the thermodynamics of spin currents in the usual normal-metal–insulating-ferromagnet interfaces are known, the results of a BKT transition are still unknown. As it is well documented, the BKT transition is associated with the unbinding of vortex-antivortex pairs in two-dimensional models with an O(2) symmetry. In our work, the ferromagnet is a layered quasi-two-dimensional material, and in the limit of weak interplane coupling, a BKT transition is expected. Using the self-consistent harmonic approximation), we have obtained the BKT transition temperature ($T_{\text{BKT}}$) and the spin current as a function of the temperature. The spin current behavior at low temperatures is similar to those obtained from theoretical and experimental systems. At $T_{\text{BKT}}$, the spin current shows a discontinuous jump associated with vortex dissociation.