Through a combination of atomistic spin-lattice dynamics simulations and relativistic ab initio calculations of electronic transport we shed light on unexplained electrical measurements in nickel nanocontacts created by break junction experiments under cryogenic conditions (4.2 K). We implement post-self-consistent-field corrections in the conductance calculations to account for spin-orbit coupling and the noncollinearity of the spins, resulting from the spin-lattice dynamics. We find that transverse magnetic domain walls are formed preferentially in (111)-oriented face-centered-cubic nickel atomic-sized contacts, which also form elongated constrictions, giving rise to enhanced individual domain wall magnetoresistance. Our calculations show that the ambiguity surrounding the conductance of a priori uniformly magnetized nickel nanocontacts can be traced back to the crystallographic orientation of the nanocontacts, rather than spontaneously formed magnetic domain walls “pinned” at their narrowest points.

In this work, the results of first-principles density-functional-theory calculations are used to construct the energy landscapes of ${\mathrm{HfO}}_2$ and its Y and Zr substituted derivatives as a function of symmetry-adapted lattice-mode amplitudes. These complex energy landscapes possess multiple local minima, corresponding to the tetragonal, oIII ($Pca{2}_1$), and oIV ($Pmn{2}_1$) phases. We find that the energy barrier between the nonpolar tetragonal phase and the ferroelectric oIII phase can be lowered by Y and Zr substitution. In ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ with an ordered cation arrangement, Zr substitution makes the oIV phase unstable, and it becomes an intermediate state in the tetragonal to oIII phase transition. Using these energy landscapes, we interpret the structural transformations and hysteresis loops computed for electric-field cycles with various choices of field direction. The implications of these results for interpreting experimental observations, such as the wake-up and split-up effects, are also discussed. These results and analysis deepen our understanding of the origin of ferroelectricity and field cycling behaviors in ${\mathrm{HfO}}_2$-based films and allow us to propose strategies for improving their functional properties.

We study ferromagnetism and its stability in twisted bilayer graphene. We work with a Hubbard-like interaction that corresponds to the screened Coulomb interaction in a well-defined limit where the Thomas-Fermi screening length $l_{\text{TF}}$ is much larger than monolayer graphene's lattice spacing $l_g\ll l_{\text{TF}}$ and much smaller than the moiré superlattice's spacing $l_{\text{TF}}\ll l_{\text{moiré}}$. We show that in the perfectly flat band “chiral” limit and at filling fractions $\pm 3/4$, the saturated ferromagnetic (spin- and valley-polarized) states are ideal ground-state candidates in the large band-gap limit. By assuming a large enough substrate (hBN) induced sublattice potential, the same argument can be applied to filling fractions $\pm 1/4$. We estimate the regime of stability of the ferromagnetic phase around the chiral limit by studying the exactly calculated spectrum of one-magnon excitations. The instability of the ferromagnetic state is signaled by a negative magnon excitation energy. This approach allows us to deform the results of the idealized chiral model (by increasing the bandwidth and/or modified interactions) toward more realistic systems. Furthermore, we use the low-energy part of the exact one-magnon spectrum to calculate the spin-stiffness of the Goldstone modes throughout the ferromagnetic phase. The calculated value of spin-stiffness can determine the excitation energy of charged skyrmions.

We study the spin-$1/2$ ferromagnetic Heisenberg-Kitaev-$\mathrm{\Gamma }$ model in the anisotropic (Toric code) limit to reveal the nature of the quantum phase transition between the gapped $Z_2$ quantum spin liquid and a spin-ordered phase (driven by Heisenberg interactions) as well as a trivial paramagnet (driven by pseudodipolar interactions $\mathrm{\Gamma }$). The transitions are obtained by a simultaneous condensation of the Ising electric and magnetic charges—the fractionalized excitations of the $Z_2$ quantum spin liquid. Both these transitions can be continuous and are examples of deconfined quantum critical points. Crucial to our calculations are the symmetry implementations on the soft electric and magnetic modes that become critical. In particular, we find strong constraints on the structure of the critical theory arising from time reversal and lattice translation symmetries with the latter acting as an anyon permutation symmetry that endows the critical theory with a manifestly self-dual structure. We find that the transition between the quantum spin liquid and the spin-ordered phase belongs to a self-dual modified Abelian Higgs field theory while that between the spin liquid and the trivial paramagnet belongs to a self-dual $Z_2$ gauge theory. We also study the effect of an external Zeeman field to show an interesting similarity between the polarized paramagnet obtained due to the Zeeman field and the trivial paramagnet driven pseudodipolar interactions. Interestingly, both the spin liquid and the spin-ordered phases have easily identifiable counterparts in the isotropic limit and the present calculations may shed insights into the corresponding transitions in the material relevant isotropic limit.

The Kondo lattice model (KLM) on the 2-dimensional triangular lattice is studied by bond fermion theory. Three-sublattice Néel order (AF) and partial Kondo screening (PKS) are considered as possible phases of the model. We find that near half filling and at moderate interaction strength, AF and PKS are energetically close and favored compared to the paramagnetic phase. We discuss the quasiparticle band structure and Fermi surface of the different phases. As is the case for the KLM on the square lattice, both the AF and the PKS phase can be further subdivided into phases with the same symmetry, but different Fermi surface topology.

Artificial hyperbolic metamaterials (HMM) are perspective for the nonlinear optical applications, their exciting functionality being due to the hyperbolic dispersion induced by a strong shape anisotropy. Here we study the second harmonic generation (SHG) in HMM formed by arrays of silver nanorods in anodic alumina. In the hyperbolic dispersion regime, giant SHG is observed associated with the fulfillment of the phase-matching conditions supported by the epsilon-near-pole resonance at the SHG wavelength, and an increase in the pump field in the epsilon-near-zero (ENZ) spectral range. We predict a strong increase in the SHG efficiency for metamaterials with the ENZ resonance at the second harmonic frequency.

We investigate the dynamical interplay between the topological state of matter and a non-Markovian dissipation, which gives rise to a crucial timescale into the system dynamics due to its quantum memory. We specifically study a one-dimensional polaronic topological superconductor with phonon-dressed $p$-wave pairing, when a fast temperature increase in surrounding phonons induces an open-system dynamics. We show that when the memory depth increases, the Majorana edge dynamics transits from relaxing monotonically to a plateau of substantial value into a collapse-and-buildup behavior, even when the polaron Hamiltonian is close to the topological phase boundary. Above a critical memory depth, the system can approach a new dressed state of the topological superconductor in dynamical equilibrium with phonons, with nearly full buildup of the Majorana correlation.

Based on the nontrivial topology of buckled antimonene, we propose two conceptual device designs based on breaking the inversion symmetry which can be achieved by gating. The first one is a quantized topological switch obtained by breaking the degeneracy of the fractionalized charge conducting edge states, inducing a metal-insulator transition. In the second one, using local gating with different polarity in different areas, a domain wall is created at their interface, which hosts a soliton type electronic gap state in which transport could be accessed by doping and shown to be robust over a wide range of disorder.

Driving a homogeneous system across a quantum phase transition in a quench time ${\tau }_Q$ generates excitations on wavelengths longer than the Kibble-Zurek (KZ) length $\widehat{\xi }\propto {\tau }_Q^{\nu /(1+z\nu )}$ within the KZ time window $\widehat{t}\propto {\tau }_Q^{z\nu /(1+z\nu )}$, where $z$ and $\nu$ are the critical exponents. Quenches designed with local time-dependent inhomogeneity can introduce a gap in the spectrum. They can be parametrized by a time- and space-dependent distance $\epsilon$ from the critical point in the parameter space of the Hamiltonian: $\epsilon (t,x)=\frac{t-x/v}{{\tau }_Q}\equiv \theta (vt-x).$ For a variety of setups with short-range interactions, they have been shown to suppress excitations if the spatial velocity $v$ of the inhomogeneous front is below the characteristic KZ velocity $\widehat{v}\propto \widehat{\xi }/\widehat{t}$. Ising-like models with long-range interactions can have no sonic horizon, spreading information instantaneously across the system. Usually, this should imply that inhomogeneous transitions will render the dynamics adiabatic regardless of the velocity of the front. However, we show that we get an adiabatic transition with no defects only when the inhomogeneous front moves slower than the characteristic crossover velocity $\tilde{v}\propto {\theta }^{(z-1)\nu /(1+\nu )}$, where $\theta$ is the slope of the inhomogeneous front at the critical point. The existence of this crossover velocity and adiabaticity of the model results from the energy gap in the quasiparticle spectrum that is opened by the inhomogeneity. This effect can be employed for efficient adiabatic quantum state preparation in systems with long-range interactions.

We study the two-dimensional extension of the Su-Schrieffer-Heeger model in its higher-order topological insulator phase, which is known to host corner states. Using the separability of the model into a product of one-dimensional Su-Schrieffer-Heeger chains, we analytically describe the eigenmodes, and specifically the zero-energy level, which includes states localized in corners. We then consider networks with disordered hopping coefficients that preserve the chiral (sublattice) symmetry of the model. We show that the corner mode and its localization properties are robust against disorder if the hopping coefficients have a vanishing flux on appropriately defined superplaquettes. We then show how this model with disorder can be realized using an acoustic network of air channels, and confirm the presence and robustness of corner modes.

Recent experiments on magnetic nanoparticle hyperthermia show that the heat dissipated by particles must be considered locally instead of characterizing it as a global quantity. Here we show theoretically that the complex energy transfer between nanoparticles interacting via magnetic dipolar fields can lead to negative local hysteresis loops and does not allow the use of these local hysteresis loops as a temperature measure. Our model shows that interacting nanoparticles release heat not only when the nanoparticle magnetization switches between different energy wells, but also in the intrawell motion, when the effective magnetic field is changed because the magnetization of another particle has switched. The temperature dynamics has a highly nontrivial dependence on the amount of precession, which is controlled by the magnetic damping. Our results constitute a step forward in modeling magnetic nanoparticles for hyperthermia and other heating applications.

We report on a pure ${}^{181}\mathrm{Ta}$ nuclear quadrupole resonance (NQR) measurement of ${\mathrm{PbTaSe}}_2$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the ${\mathrm{TaSe}}_2$ layer. We observed the ${}^{181}\mathrm{Ta}$ NQR spectrum of the intrinsic structure with space group $P\overline{6}m2$, which agrees well with density functional theory calculations. The nuclear spin relaxation rate $1/T_1$ shows an exponential decrease well below $T_{{}_{\mathrm{c}}}$, indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by ${}^{181}\mathrm{Ta}$ NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of $\mathrm{Ta}5d$ orbitals, where the average pairing interactions are expected to be weaker than in the BCS model, are primarily probed. The temperature dependence of $1/T_1$ below $T_{{}_{\mathrm{c}}}$ can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of the superconducting gap size inherent in multiple Fermi surfaces theoretically proposed in ${\mathrm{PbTaSe}}_2$.

Phase transitions have recently been formulated in the time domain of quantum many-body systems, a phenomenon dubbed dynamical quantum phase transitions (DPTs), whose phenomenology is often divided in two types. One refers to distinct phases according to long-time averaged order parameters, while the other is focused on the nonanalytical behavior emerging in the rate function of the Loschmidt echo. Here we show that such DPTs can be found in systems with few degrees of freedom; i.e., they can take place without resorting to the traditional thermodynamic limit. We illustrate this by showing the existence of the two types of DPTs in a quantum Rabi model—a system involving a spin-$\frac{1}{2}$ and a bosonic mode. The dynamical criticality appears in the limit of an infinitely large ratio of the spin frequency with respect to the bosonic one. We determine its dynamical phase diagram and study the long-time averaged order parameters, whose semiclassical approximation yields a jump at the transition point. We find the critical times at which the rate function becomes nonanalytical, showing its associated critical exponent as well as the corrections introduced by a finite frequency ratio. Our results open the door for the study of DPTs without the need to scale up the number of components, thus allowing for their investigation in well controllable systems.

Optimal parameter settings for application problems embedded into hardware graphs is key to practical quantum annealers (QAs). Embedding chains typically crop up as harmful Griffiths phases but can be used as a resource as we show here: to balance out singularities in the logical problem, changing its universality class. A smart choice of embedding parameters reduces annealing times for random Ising chain from $O(exp\left[c\sqrt{N}\right])$ to $O\left(N^2\right)$. Dramatic reduction in time-to-solution for QAs is confirmed by numerics, for which we developed a custom integrator to overcome convergence issues.

Employing first-principles density functional theory calculations and Wannierization of the low-energy band structure, we analyze the electronic structure of undoped, infinite-layer nickelate compounds ${\mathrm{NdNiO}}_2,$ ${\mathrm{PrNiO}}_2,$ and ${\mathrm{LaNiO}}_2$. Our study reveals the important role of the nonzero $f$-ness of Nd and Pr atoms, as opposed to the $f^0$ occupancy of La. The nonzero $f$-ness becomes effective in lowering the energy of the rare-earth $5d$ hybridized axial orbital, thereby enhancing the electron pockets and influencing the Fermi surface topology. The Fermi surface topology of ${\mathrm{NdNiO}}_2$ and ${\mathrm{PrNiO}}_2$ is strikingly similar, while differences are observed for ${\mathrm{LaNiO}}_2$. This difference shows up in computed doping-dependent superconducting properties of the three compounds within a weak coupling theory, which finds two-gap superconductivity for ${\mathrm{NdNiO}}_2$ and ${\mathrm{PrNiO}}_2$, and the possibility of a single-gap superconductivity for ${\mathrm{LaNiO}}_2$ with the strength of superconductivity suppressed by almost a factor of 2, compared to the Nd or Pr compound.

Hard magnetic nanocomposites are attractive materials for integration in various microsystems and for building of next-generation permanent magnets. However, exploiting their full potential requires control of their microstructure at the nanometer scale. Studying these materials in model systems synthesized by nanofabrication routes provides interesting insights into the interplay between the microstructure and the magnetic performances. Here, by using a combination of mass-selected low-energy cluster beam deposition and electron-beam evaporation, we prepare nanocomposite films where Co nanoinclusions are integrated in a hard magnetic FePt matrix. Local atomic structures and element-selective magnetic properties of such nanocomposites have been thoroughly investigated using polarization-dependent hard x-ray absorption spectroscopies. These results demonstrate that magnetically soft inclusions are stabilized at room temperature, emphasizing the role of interdiffusion in the preparation of nanocomposites.

We investigate the interplay between spatial anisotropy and further exchange interactions in the spin-$\frac{1}{2}$ Heisenberg antiferromagnetic model on a triangular lattice. We use the Schwinger boson theory by including Gaussian fluctuations above the mean-field approach. The phase diagram exhibits a strong reduction of the long-range collinear and incommensurate spiral regions with respect to the mean-field ones. This reduction is accompanied by the emergence of its short-range order counterparts, leaving an ample room for zero-flux and nematic spin-liquid regions. Remarkably, within the neighborhood of the spatially isotropic line, there is a range where the spirals are so fragile that only the commensurate ${120}^{\circ }$ Néel ones survive. The good agreement with recent variational Monte Carlo predictions gives support to the rich phase diagram induced by spatial anisotropy.

Polar dielectrics are a promising platform for midinfrared nanophotonics, allowing for nanoscale electromagnetic energy confinement in oscillations of the crystal lattice. We recently demonstrated that in nanoscopic polar systems a local description of the optical response fails, leading to erroneous predictions of modal frequencies and electromagnetic field enhancements. In this paper we extend our previous work providing a scattering matrix theory of the nonlocal optical response of planar, anisotropic, layered polar dielectric heterostructures. The formalism we employ allows for the calculation of both reflection and transmission coefficients, and of the guided mode spectrum. We apply our theory to complex AlN/GaN superlattices, demonstrating the strong nonlocal tunability of the optical response arising from hybridization between photon and phonon modes. The numerical code underlying these calculations is provided in an online repository to serve as a tool for the design of phonon-based midinfrared optoelectronic devices.