We investigate the ultrafast dynamics of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$, a prototype material showing a Lifshitz-type transition as a function of temperature. In the paramagnetic metallic state, the photoreflectivity shows a subpicosecond relaxation, followed by a featureless small offset. In the antiferromagnetic state slightly below $T_N$, however, the photoreflectivity resurges over hundreds of picoseconds, which goes beyond the usual realm of the effective-temperature model. Our observations are consistent with the Lifshitz phase transition of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ driven by the evolution of the local magnetic moment.

Structural and magnetic properties of a quasi-one-dimensional spin-$\frac{1}{2}$ compound ${\mathrm{NaVOPO}}_4$ are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of ${\mathrm{NaVOPO}}_4$ may be compatible with the gapless uniform spin chain model, detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio $\alpha \simeq 0.98$ and the ensuing zero-field spin gap ${\mathrm{\Delta }}_0/k_{\mathrm{B}}\simeq 2.4\mathrm{K}$ directly probed by field-dependent magnetization measurements. No long-range order is observed down to 50 mK in zero field. However, applied fields above the critical field $H_{c1}\simeq 1.6\mathrm{T}$ give rise to a magnetic ordering transition with the phase boundary $T_N\propto {(H-H_{c1})}^{\frac{1}{\varphi }}$, where $\varphi \simeq 1.8$ is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, ${\mathrm{NaVOPO}}_4$ lies close to the quantum critical point.

We report the results of an unpolarized small-angle neutron-scattering (SANS) study on Mn-Zn ferrite (MZFO) magnetic nanoparticles with the aim to elucidate the interplay between their particle size and the magnetization configuration. We study different samples of single-crystalline MZFO nanoparticles with average diameters ranging between 8 to 80 nm, and demonstrate that the smallest particles are homogeneously magnetized. However, with increasing nanoparticle size, we observe the transition from a uniform to a nonuniform magnetization state. Field-dependent results for the correlation function confirm that the internal spin disorder is suppressed with increasing field strength. The experimental SANS data are supported by the results of micromagnetic simulations, which confirm an increasing inhomogeneity of the magnetization profile of the nanoparticle with increasing size. The results presented demonstrate the unique ability of SANS to detect even very small deviations of the magnetization state from the homogeneous one.

Magnetic skyrmions are particlelike chiral spin textures found in magnetic films with out-of-plane anisotropy and are considered to be potential candidates as information carriers in next generation data storage devices. Despite intense research into the nature of skyrmions and their dynamic properties, there are several key challenges that still need to be addressed. In particular, the outstanding issues are the reproducible generation, stabilization, and confinement of skyrmions at room temperature. Here, we present a method for the capture of magnetic skyrmions in an array of defects in the form of an antidot lattice. We find that inhomogeneity in the total effective field produced by the antidot lattice is important for the formation of skyrmions which are mainly stabilized by the dipolar interaction. With micromagnetic simulations and scanning transmission x-ray microscopy we elucidate that the formation of skyrmions within the antidot lattice depends on the lattice constant and that, below a certain lattice constant, the skyrmion formation is suppressed. Based on our results we propose that, by varying the lattice constant, we can modify the probability of skyrmion formation in different parts of a sample by specific patterning. This provides another platform for experimental investigations of skyrmions and skyrmion-based devices.

The magnetically frustrated spin ice family of materials is host to numerous exotic phenomena such as magnetic monopole excitations and macroscopic residual entropy extending to low temperature. A finite-temperature ordering transition in the absence of applied fields has not been experimentally observed in the classical spin ice materials ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Such a transition could be induced by the application of pressure, and in this work we consider the effects of uniaxial pressure on classical spin ice systems. Theoretically, we find that the pressure-induced ordering transition in ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is strongly affected by the dipolar interaction. We also report measurements of the neutron structure factor of ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ under pressure and compare the experimental results to the predictions of our theoretical model.

A meta-generalized gradient approximation (meta-GGA) exchange functional, based only on the Pauli kinetic enhancement factor, is proposed for quantum chemistry and solid-state physics. The underlining construction of this exchange energy functional is based on a nonlinear equation found from a generalization of the Becke-Roussel approach, where we use the cuspless hydrogen exchange hole density model. We combine the meta-GGA exchange with a simple GGA correlation. The resulting exchange-correlation semilocal functional is remarkably accurate for several thermochemical test cases and quite efficient for the structural and energetic properties of bulk solids. It also provides, in the generalized Kohn-Sham scheme, better orbital energies of atoms and band gaps of semiconductors than other popular semilocal functionals. The present construction is showing the high relevance of the Pauli kinetic energy density for the functional development.

We report on ultrafast time-resolved pump-probe studies in a CdZnTe/CdMgTe planar guiding structure covered with a metallic grating. The one-dimensional periodic gold structure allows for efficient coupling into the guiding layer for $p$-polarized 30 fs optical pulses with a large spectral bandwidth of about 60 nm. The resulting spectral width of optical pulses propagating inside the guiding layer corresponds to 20–30 nm. We demonstrate that the excitation of exciton-polariton modes in the guiding layer leads to a modulation of the optical response in the vicinity of the excitonic resonance. Spatially resolved pump-probe measurements show an asymmetric behavior in the optical response when the relative position of the pump and probe spots is varied on the scale of ten micrometers perpendicular to the metal ridges. This is attributed to the excitation of resonant and off-resonant exciton-polariton modes which propagate in opposite directions inside the guiding layer in accordance with their dispersion relations. Two main mechanisms are considered and evaluated, namely, Pauli blocking and excitation-induced dephasing, which are shown to be responsible for the pump-induced changes in the exciton absorption spectrum. While both of these processes lead to the generation of photoexcited carriers in the guiding layer, their impact on the optical properties (transmission and reflection) are different which leads to the asymmetric behavior of the spatially resolved transients.

This work shows that a strongly correlated phase which is gapped to collective spin excitations but gapless to charge fluctuations emerges as a universal feature in one-dimensional fermionic systems obeying certain discrete symmetries whenever one pair of spin-degenerate subbands is occupied and an arbitrarily weak spin-orbit interaction is present. This general result is independent of the details of the one-dimensional confinement, the fermionic spin, or nature of the spin-orbit interaction. In narrow-gap semiconductors, this gap may be of order 10 µeV. This strongly correlated phase may be identified both via an anomalous $h/2e$ flux periodicity in Aharonov-Bohm oscillations and $2e$ periodic Coulomb blockade, features which reflect the existence of fermionic pairing despite the absence of superconductivity and the repulsive nature of the interaction.

Valleytronics is an emerging field of research which employs energy valleys in the band structure of two-dimensional (2D) electronic materials to encode information. A special interest has been triggered by the associated spin-valley coupling which reveals rich fundamental physics and enables new functionalities. Here, we propose exploiting the spin-valley locking in 2D materials with a large spin-orbit coupling and electric-field reversible valley spin polarization, such as germanene, stanene, a $1T^{\prime }$ transition metal dichalcogenide (TMDC) monolayer, and a $2H$-TMDC bilayer, to realize a valley spin valve (VSV). The valley spin polarization in these materials can be switched by an external electric field, which enables functionalities of a valley spin polarizer or a valley spin analyzer. When placed in series, they constitute the proposed VSV—a device whose conductance state is ON or OFF depending on the relative valley spin polarization of the polarizer and the analyzer. Using quantum-transport calculations based on an adequate tight-binding model, we predict a giant VSV ratio of nearly 100% for both germanene- and stanene-based VSV devices. Our results demonstrate the implication of the spin-valley coupling in 2D materials for the novel device concept promising for valleytronics.

A metal near the topological transition can be loosely viewed as consisting of two groups of electrons. The first group are “bulk” electrons occupying most of the Brillouin zone. The second group are electrons with wave vectors close to the topological transition point. Kinetic energy, ${\tilde{E}}_F$, of electrons of the first group is much bigger than kinetic energy, $E_F$, of electrons of the second group. With electrons of the second group being slow, the interaction effects are more pronounced for these electrons. We perform a calculation illustrating that electrons of the second group are responsible for inelastic lifetime making it anomalously short, so the concept of quasiparticles applies to these electrons only marginally. We also demonstrate that interactions renormalize the spectrum of electrons in the vicinity of topological transition, the parameters of renormalized spectrum being strongly dependent on the proximity to the transition. Another many-body effect that evolves dramatically as the Fermi level is swept through the transition is the Friedel oscillations of the electron density created by electrons of the second group around an impurity. These oscillations are strongly anisotropic with a period depending on the direction. Scattering of electrons off these oscillations give rise to a temperature-dependent ballistic correction to the conductivity.

Recent progress has witnessed that various topological physics can be simulated by electric circuits under alternating current. However, it is still a nontrivial problem if it is possible to simulate the dynamics subject to the Schrödinger equation based on electric circuits. In this work, we reformulate the Kirchhoff law in one dimension in the form of the Schrödinger equation. As a typical example, we investigate quantum walks in $LC$ circuits. We also investigate how quantum walks are different in topological and trivial phases by simulating the Su-Schrieffer-Heeger model in electric circuits. We then generalize them to include dissipation and nonreciprocity by introducing resistors, which produce non-Hermitian effects. We point out that the time evolution of one-dimensional quantum walks is exactly solvable with the use of the generating function made of the Bessel functions.

We investigate hybrid structures based on a bilayer quantum spin Hall system in proximity to an $s$-wave superconductor as a platform to mimic time-reversal symmetric topological superconductors. In this bilayer setup, the induced pairing can be of intra- or interlayer type, and domain walls of those different types of pairing potentials host Kramers partners (time-reversal conjugate pairs) of Majorana bound states. Interestingly, we discover that such topological interfaces providing Majorana bound states can also be achieved in an otherwise homogeneous system by a spatially dependent interlayer gate voltage. This gate voltage causes the relative electron densities of the two layers to vary accordingly, which suppresses the interlayer pairing in regions with strong gate voltage. We identify particular transport signatures (zero-bias anomalies) in a five-terminal setup that are clearly related to the presence of Kramers pairs of Majorana bound states.

In this work we combine density functional theory and quantum transport calculations to study the influence of atomic-scale defects on the work function and field emission characteristics of metal surfaces. We develop a general methodology for the calculation of the field emitted current density from nanofeatured surfaces, which is then used to study specific defects on a Cu(111) surface. Our results show that the inclusion of a defect can significantly locally enhance the field emitted current density. However, this increase is attributed solely to the decrease of the work function due to the defect, with the effective field enhancement being minute. Finally, the Fowler-Nordheim equation is found to be valid when the modified value for the work function is used, with only an approximately constant factor separating the computed currents from those predicted by the Fowler-Nordheim equation.

We study the low-energy physics of a chain of Majorana fermions in the presence of interaction and disorder, emphasizing the difference between Majoranas and conventional (complex) fermions. While in the noninteracting limit both models are equivalent (in particular, belong to the same symmetry class BDI and flow towards the same infinite-randomness critical fixed point), their behavior differs drastically once interaction is added. Our density-matrix renormalization group calculations show that the complex-fermion chain remains at the noninteracting fixed point. On the other hand, the Majorana fermion chain experiences a spontaneous symmetry breaking and localizes for repulsive interaction. To explain the instability of the critical Majorana chain with respect to a combined effect of interaction and disorder, we consider interaction as perturbation to the infinite-randomness fixed point and calculate numerically two-wave-function correlation functions that enter interaction matrix elements. The numerical results supported by analytical arguments exhibit a rich structure of critical eigenstate correlations. This allows us to identify a relevant interaction operator that drives the Majorana chain away from the infinite-randomness fixed point. For the case of complex fermions, the interaction is irrelevant.

Superconducting Weyl semimetals present a novel and promising system to harbor new forms of unconventional topological superconductivity. Within the context of time-reversal-symmetric Weyl semimetals with $d$-wave superconductivity, we demonstrate that the number of Majorana cones equates to the number of intersections between the $d$-wave nodal lines and the Fermi arcs. We illustrate the importance of nodal line-arc intersections by demonstrating the existence of locally stable surface Majorana cones that the winding number does not predict. The discrepancy between Majorana cones and the winding number necessitates an augmentation of the winding number formulation to account for each intersection. In addition, we show that imposing additional mirror symmetries globally protects the nodal line-arc intersections and the corresponding Majorana cones.

In twisted bilayer graphene, long-wavelength lattice fluctuations on the scale of the moiré period are dominated by phason modes, i.e., acoustic branches of the incommensurate lattice resulting from coherent superpositions of optical phonons. In the limit of small twist angles, these modes describe the sliding motion of stacking domain walls separating regions of partial commensuration. The resulting soliton network is a soft elastic manifold, whose reduced rigidity arises from the competition between intralayer (elastic) and interlayer (adhesion) forces governing lattice relaxation. Shear deformations of the beating pattern dominate the electron-phason coupling to the leading order in $t_{\perp }/t$, the ratio between interlayer and intralayer hopping parameters. This coupling lifts the layer degeneracy of the Dirac cones at the corners of the moiré Brillouin zone, which could explain the observed fourfold (instead of eightfold) Landau level degeneracy. Electron-phason scattering gives rise to a linear-in-temperature contribution to the resistivity that increases with decreasing twist angle due to the reduction of the stiffness of the soliton network. This contribution alone, however, seems to be insufficient to explain the huge enhancement of the resistivity of the normal state close to the magic angle.

Low temperature measurements of the tunneling current as a function of the applied bias voltage have been performed on a dense constant-height grid above individual copper phthalocyanine molecules adsorbed on a Cu(111) surface. By appropriate tuning of the applied bias, the molecule can be reversibly switched between two configurations in which pairs of opposite maxima appear rotated by ${90}^{\circ }$ in the tunneling current map. The underlying conformations are revealed by density functional calculations including van der Waals interactions: a $C_{2v}$ symmetric ground state and two energetically equivalent states, in which the molecule is twisted and rotated around its center by $\pm 7^{\circ }$. For tip biases above 200 mV position-dependent current switching is observed, as in previous measurements of telegraph noise [Schaffert et al., Nat. Mater. 12, 223 (2013)]. In a small voltage interval around zero the measured current becomes bistable. Switching to a particular state can be initiated by sweeping the voltage past well-defined positive and negative thresholds at certain positions above the molecule or by scanning at constant current and a reduced reverse bias.