We study the dynamical thermal conductivity of the two-dimensional Kitaev spin model on the honeycomb lattice. We find a strongly temperature dependent low-frequency spectral intensity as a direct consequence of fractionalization of spins into mobile Majorana matter and a static ${\mathbb{Z}}_2$ gauge field. The latter acts as an emergent thermally activated disorder, leading to the appearance of a pseudogap which closes in the thermodynamic limit, indicating a dissipative heat conductor. Our analysis is based on complementary calculations of the current correlation function, comprising exact diagonalization by means of a complete summation over all gauge sectors, as well as a phenomenological mean-field treatment of thermal gauge fluctuations, valid at intermediate and high temperatures. The results will also be contrasted against the conductivity discarding gauge fluctuations.

We have performed a joint study of magnetization and magnetoresistance in the antiferromagnetic (AFM) semimetal NdSb under pulsed high magnetic field up to 60 T. We show that NdSb displays extremely large magnetoresistance (XMR), reaching values of $1.77\times {10}^6\%$ at 1.3 K and 60 T without any sign of saturation. The field-dependent magnetization at 4.2 K reveals a field-induced metamagnetic transition from the AFM to the ferromagnetic state around 9 T. During the metamagnetic transition, a steplike reduction of resistance occurs, resembling the giant magnetoresistance (GMR) behavior in metallic multilayers. The investigation of the Shubnikov–de Haas effect in NdSb suggests that a particular orbital texture might contribute to the XMR effect in addition to the perfect electron-hole compensation.

Electrical transport in double quantum dots (DQDs) illuminates many interesting features of the dots' carrier states. Recent advances in silicon quantum information technologies have renewed interest in the valley states of electrons confined in silicon. Here we show measurements of dc transport through a mesa-etched silicon double quantum dot. Comparing bias triangles (i.e., regions of allowed current in DQDs) at positive and negative bias voltages we find a systematic asymmetry in the size of the bias triangles at the two bias polarities. Asymmetries of this nature are associated with blocked tunneling events due to the occupation of a metastable state. Several features of our data lead us to conclude that the states involved are not simple spin states. Rather, we develop a model based on selective filling of valley states in the DQD that is consistent with all of the qualitative features of our data.

We propose a universal mechanism that makes it possible to drive an individual atomic spin using a spin-polarized scanning tunnel microscope (STM) with an oscillating electric signal. We show that the combination of the distance-dependent exchange with the magnetic tip and the electrically driven mechanical oscillation of the surface spins permits us to control their quantum state. Based on a combination of density functional theory and multiplet calculations, we show that the proposed mechanism is essential to account for the recently observed electrically driven paramagnetic spin resonance (ESR) of an individual Fe atom on a MgO/Ag(100) surface. Our findings set the foundation to deploy the ESR-STM quantum sensing technique to a much broader class of systems.

This paper proposes the method to analyze the internal dynamics of nanoscopic systems by periodic modulation of the electrochemical potentials of the attached leads and measuring the time-averaged current. The idea is presented using the example of a double quantum dot coupled to one nonmagnetic and one spin-polarized lead. The current flowing through the molecule is shown to depend on both the frequency of the modulation and the exchange coupling between the electrons occupying the molecule. In particular, one can observe a pronounced oscillatory behavior of the current-frequency dependence, which reveals the coherent oscillations between the spin states of the system.

We investigate the effects of Fe and Co substitutions on the phase stability of the martensitic phase and mechanical, electronic, and magnetic properties of the magnetic shape memory system ${\mathrm{Mn}}_2\mathrm{NiGa}$ by first-principles density functional theory calculations. The evolution of these aspects upon substitution of Fe and Co at different crystallographic sites is investigated by computing the electronic structure, mechanical properties (tetragonal shear constant, Pugh ratio, and Cauchy pressure), and magnetic exchange parameters. We find that the austenite phase of ${\mathrm{Mn}}_2\mathrm{NiGa}$ gradually stabilizes with increase in concentration of Fe/Co due to the weakening of the minority spin hybridization of Ni and Mn atoms occupying crystallographically equivalent sites. The interplay between relative structural stability and the compositional changes is understood from the variations in the elastic moduli and electronic structures. We find that like in the ${\mathrm{Ni}}_2\mathrm{MnGa}$-based systems, the elastic shear modulus $C^{\prime }$ can be considered as a predictor of composition dependence of martensitic transformation temperature $T_m$ in substituted ${\mathrm{Mn}}_2\mathrm{NiGa}$, thus singling it out as the universally acceptable predictor for martensitic transformation in Ni-Mn-Ga compounds over a wide composition range. The magnetic properties of ${\mathrm{Mn}}_2\mathrm{NiGa}$ are found to be greatly improved by the substitutions due to stronger ferromagnetic interactions in the compounds. The gradually weaker (stronger) Jahn-Teller distortion (covalent bonding) in the minority spin densities of states due to substitutions leads to a half-metallic-like gap in these compounds resulting in materials with high spin polarization when the substitutions are complete. The substitutions at the Ga site result in the two compounds ${\mathrm{Mn}}_2\mathrm{NiFe}$ and ${\mathrm{Mn}}_2\mathrm{NiCo}$ with very high magnetic moments and Curie temperatures. Thus, our work indicates that although the substitutions destroy the martensitic transformation and thus the possibility of realization of shape memory properties in ${\mathrm{Mn}}_2\mathrm{NiGa}$, magnetic materials with very good magnetic parameters that are potentially useful for novel magnetic applications can be obtained. This can trigger interest in the experimental community in further research on substituted ${\mathrm{Mn}}_2\mathrm{NiGa}$.

Neutron and x-ray scattering measurements were performed on $\left(\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3\text{−}x\mathrm{at}\%\mathrm{BaTi}{\mathrm{O}}_3$ (NBT-$x\mathrm{BT}$) single crystals ($x=4$, 5, 6.5, and 7.5) across the morphotropic phase boundary (MPB), as a function of both composition and temperature, and probing both structural and dynamical aspects. In addition to the known diffuse scattering pattern near the $\mathrm{\Gamma }$ points, our measurements revealed new, faint superlattice peaks, as well as an extensive diffuse scattering network, revealing a short-range ordering of polar nanoregions (PNR) with a static stacking morphology. In samples with compositions closest to the MPB, our inelastic neutron scattering investigations of the phonon dynamics showed two unusual features in the acoustic phonon branches, between the superlattice points, and between the superlattice points and $\mathrm{\Gamma }$ points, respectively. These critical elements are not present in the other compositions away from the MPB, which suggests that these features may be related to the tilt modes coupling behavior near the MPB.

The state of metastable equilibrium glass can carry an imprint of the past and exhibit memory effect. As a hallmark of glassy dynamics, memory effect can affect glassy behavior as it evolves further upon time. Even though the physical picture of the memory effect has been well studied, it is unclear whether a glass can recall as many pieces of information as possible, and if so, how the glass will accordingly behave. We report that by fractionizing temperature interval, inserting multistep aging protocols, and optimizing the time of each temperature step, i.e., by imposing a multistep “training” on a prototypical $\mathrm{P}{\mathrm{d}}_{40}\mathrm{N}{\mathrm{i}}_{10}\mathrm{C}{\mathrm{u}}_{30}{\mathrm{P}}_{20}$ metallic glass, the memory of the trained glass can be significantly strengthened, marked by a pronounced augment in potential energy. These findings provide a new guide for regulating the energy state of glass by enhancing the nonequilibrium behaviors of the memory effect and offer an opportunity to develop a clearer physical picture of glassy dynamics.

A force-matched spline-based empirical potential based on the modified embedded-atom method is fit to an extensive database of highly converged density functional theory calculations for titanium-niobium alloys with an evolutionary algorithm. Consistency with experiment and existing models is verified through calculations of structural, elastic, and thermal properties. The potential is used to study the effects of alloying on elastic properties, the martensite phase of Ti-rich alloys, and the $\alpha \text{−}\beta$ and $\beta \text{−}\omega$ transformations. Stress- and temperature-induced martensitic transitions are demonstrated in Ti-25 at. % Nb by NPT simulation. Last, the effect of alloying on screw dislocations in hcp-Ti and bcc-Nb is analyzed at 5 at. % solute content with the fitted potential.

Starting from high-level ab initio calculations, we present ultrafast spin dynamical effects and all-spin-based nanologic elements on a ${\mathrm{Ni}}_4$ cluster. The predominant underlying mechanisms for our operations are optically induced coherent $\mathrm{\Lambda }$ processes, the optimized parameters of which are obtained from a dedicated genetic algorithm search. The geometry of the cluster exhibits a high degree of spin localization. Thus, maneuvering the spin localizations we construct a pure-spin OR gate. Additionally, functional intramolecular cooperative effects such as spin bifurcation and spin merging are introduced that facilitate the setup of the latter. In the which-path information effect the phase of the final spin state is exploited to reveal the path traveled by the spin in a two-step spin transfer scenario. This is beneficial for both classical and quantum computations on small spatial and temporal scales.

We propose theoretically an effective scheme for braiding Majorana bound states by manipulating the point potential. The vortex pinning effect is carefully elucidated. This effect can be used to control the vortices and Majorana bound states in topological superconductors. The exchange of two vortices induced by moving the potentials is simulated numerically. The zero-energy state in the vortex core is robust with respect to the strength of the potential. The Majorana bound states in a pinned vortex are identified numerically.

The structural properties of ${\mathrm{LaRu}}_2{\mathrm{P}}_2$ under external pressure have been studied up to 14 GPa, employing high-energy x-ray diffraction in a diamond-anvil pressure cell. At ambient conditions, ${\mathrm{LaRu}}_2{\mathrm{P}}_2$ (I4/mmm) has a tetragonal structure with a bulk modulus of $B=105\left(2\right)$ GPa and exhibits superconductivity at $T_c=4.1$ K. With the application of pressure, ${\mathrm{LaRu}}_2{\mathrm{P}}_2$ undergoes a phase transition to a collapsed tetragonal (cT) state with a bulk modulus of $B=175\left(5\right)$ GPa. At the transition, the $c$-lattice parameter exhibits a sharp decrease with a concurrent increase of the $a$-lattice parameter. The cT phase transition in ${\mathrm{LaRu}}_2{\mathrm{P}}_2$ is consistent with a second-order transition, and was found to be temperature dependent, increasing from $P=3.9\left(3\right)$ GPa at 160 K to $P=4.6\left(3\right)$ GPa at 300 K. In total, our data are consistent with the cT transition being near, but slightly above 2 GPa at 5 K where superconductivity is suppressed. Finally, we compare the effect of physical and chemical pressure in the R${\mathrm{Ru}}_2{\mathrm{P}}_2$ (R = Y, $\mathrm{La}–\mathrm{Er}$, Yb) isostructural series of compounds and find them to be analogous.

Periodically driven (Floquet) systems have been under active theoretical and experimental investigations. This paper aims at a systematic study in the following aspects of Floquet systems: (i) a systematic formulation of topological invariants of Floquet systems based on the cooperation of topology and symmetries. Topological invariants are constructed for the ten symmetry classes in all spatial dimensions, for both homogeneous Floquet systems (Floquet topological insulators and superconductors) and Floquet topological defects. Meanwhile, useful representative Dirac Hamiltonians for all the symmetry classes are obtained and studied. (ii) A general theory of Floquet topological defects, based on the proposed topological invariants. (iii) Models and proposals of Floquet topological defects in low dimensions. Among other defect modes, we investigate Floquet Majorana zero modes and Majorana Pi modes in vortices of topologically trivial superconductors under a periodic drive. In addition, we clarified several notable issues about Floquet topological invariants. Among other issues, we prove the equivalence between the effective-Hamiltonian-based band topological invariants and the frequency-domain band topological invariants.

Recently determined atomistic scale structures of near-two dimensional bilayers of vitreous silica (using scanning probe and electron microscopy) allow us to refine the experimentally determined coordinates to incorporate the known local chemistry more precisely. Further refinement is achieved by using classical potentials of varying complexity: one using harmonic potentials and the second employing an electrostatic description incorporating polarization effects. These are benchmarked against density functional calculations. Our main findings are that (a) there is a symmetry plane between the two disordered layers, a nice example of an emergent phenomena, (b) the layers are slightly tilted so that the Si-O-Si angle between the two layers is not ${180}^{\circ }$ as originally thought but rather $175\pm 2^{\circ }$, and (c) while interior areas that are not completely imagined can be reliably reconstructed, surface areas are more problematic. It is shown that small crystallites that appear are just as expected statistically in a continuous random network. This provides a good example of the value that can be added to disordered structures imaged at the atomic level by implementing computer refinement.

We develop a method to characterize topological phase transitions for strongly correlated Hamiltonians defined on two-dimensional lattices based on the many-body Berry curvature. Our goal is to identify a class of quantum critical points between topologically nontrivial phases with fractionally quantized Hall (FQH) conductivity and topologically trivial gapped phases through the discontinuities of the many-body Berry curvature in the so-called flux Brillouin zone (fBZ), the latter being defined by imposing all possible twisted boundary conditions. For this purpose, we study the finite-size signatures of several quantum phase transitions between fractional Chern insulators and charge-ordered phases for two-dimensional lattices by evaluating the many-body Berry curvature numerically using exact diagonalization. We observe degeneracy points (nodes) of many-body energy levels at high-symmetry points in the fBZ, accompanied by diverging Berry curvature. We find a correspondence between the number and order of these nodal points, and the change of the topological invariants of the many-body ground states across the transition, in close analogy with Weyl nodes in noninteracting band structures. This motivates us to apply a scaling procedure, originally developed for noninteracting systems, for the Berry curvature at the nodal points. This procedure offers a useful tool for the classification of topological phase transitions in interacting systems harboring FQH like topological order.

We consider layered decorated honeycomb lattices at two-thirds filling, as realized in some trinuclear organometallic complexes. Localized $S=1$ moments with a single-spin anisotropy emerge from the interplay of Coulomb repulsion and spin molecular-orbit coupling (SMOC). Magnetic anisotropies with bond-dependent exchange couplings occur in the honeycomb layers when the direct intracluster exchange and the spin molecular-orbital coupling are both present. We find that the effective spin exchange model within the layers is an XXZ + ${120}^{\circ }$ honeycomb quantum compass model. The intrinsic nonspherical symmetry of the multinuclear complexes leads to very different transverse and longitudinal spin molecular-orbital couplings, which greatly enhances the single-spin and exchange coupling anisotropies. The interlayer coupling is described by an XXZ model with anisotropic biquadratic terms. As the correlation strength increases the system becomes increasingly one-dimensional. Thus, if the ratio of SMOC to the interlayer hopping is small this stabilizes the Haldane phase. However, as the ratio increases there is a quantum phase transition to the topologically trivial “$D$ phase.” We also predict a quantum phase transition from a Haldane phase to a magnetically ordered phase at sufficiently strong external magnetic fields.

The Casimir force between macroscopic bodies is well understood, but not the Casimir stress inside bodies. Suppose empty space or a uniform medium meets a soft wall where the refractive index is continuous but its derivative jumps. For this situation we predict a characteristic power law for the stress inside the soft wall and close to its edges. Our result shows that such edges are not tolerated in the aggregation of liquids at surfaces, regardless whether the liquid is attracted or repelled.