Nonconservative current-induced forces (NCIFs) in nanowires have attracted attention both as a failure mechanism and as a driver for atomic-scale motors. Here we investigate the view that the nonconservative character of the electron wind force is an intrinsic property of the electron flow. To this end we consider the curl of the force on a noninvasive test scatterer immersed in the electron current. Using a free-electron Hamiltonian, we find nonzero curls at the opening of conductance channels due to variations in the local density of states. Current vortices are observed in these simulations, and one would naively expect them to give rise to large curls. However, this is not always the case. We find two types, low- and high-curl current vortices. Bouncy patterns, the ‘snaking’ of the current flow through the wire, are a prominent effect that gives rise to large curls. Ultimately NCIFs reduce the stability of nanowires and therefore it is crucial to study their origin to manage their effects. This approach enhances our understanding of NCIFs by going beyond forces on individual nuclei.

Here, we present a study on the Fermi surface of the Dirac type-II semimetallic candidate $\mathrm{Ni}{\mathrm{Te}}_2$ via the temperature and angular dependence of the de Haas–van Alphen (dHvA) effect measured in single crystals grown through Te flux. In contrast to its isostructural compounds like $\mathrm{Pt}{\mathrm{Se}}_2$, band-structure calculations predict $\mathrm{Ni}{\mathrm{Te}}_2$ to display a tilted Dirac node very close to its Fermi level that is located along the $\mathrm{\Gamma }$ to $A$ high-symmetry direction within its first Brillouin zone. The angular dependence of the dHvA frequencies is found to be in agreement with the first-principles calculations when the electronic bands are slightly shifted with respect to the Fermi level (${\varepsilon }_F$), and therefore provide support for the existence of a Dirac type-II node in $\mathrm{Ni}{\mathrm{Te}}_2$. Nevertheless, we observed mild disagreements between experimental observations and density functional theory (DFT) calculations as, for example, nearly isotropic and light experimental effective masses. This indicates that the dispersion of the bands is not well captured by DFT. Despite the coexistence of Dirac-like fermions with topologically trivial carriers, samples of the highest quality display an anomalous and large either linear or sublinear magnetoresistivity. This suggests that Lorentz invariance breaking Dirac-like quasiparticles dominate the carrier transport in this compound.

The elastic modulus $c_{44}$ of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. We also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes, respectively.

There are two discrepant experimental results on the magnetic ground state of ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$; one study proposes a spin-glass state, while the other introduces the material as a ferromagnet. In this study, we attempt to resolve this issue by employing density functional theory and Monte Carlo simulations. We derive different spin models by varying the Hubbard $U$ parameter in ab initio $\mathrm{GGA}+U$ calculations. For most of the range of Hubbard $U$, we find that the leading terms in the spin Hamiltonian are biquadratic and nearest-neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations of these models with the experiments, we find a ferromagnetic ground state for ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ to be the most compatible with experiments. We also consider ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ as a prototype of the defect-free pyrochlore system with spin-glass behavior and compare it with ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$. The orbital degrees of freedom are considered as a leading factor in converting a defect-free pyrochlore such as ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ to a spin-glass system. By changing the $d$ orbital occupations of Mo atoms, our $\mathrm{GGA}+U$ calculations for ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ indicate many nearly degenerate states with different $d$ orbital orientations, which reveals $d$ orbital degrees of freedom in this material. However, for ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$, we find a single ground state with a fixed orbital orientation. Consequently, all of our ab initio approaches confirm ${\mathrm{Y}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ is a ferromagnetic system.

The energy levels of organic semiconductors are primarily determined by the molecular orbital energies of constituent molecules. Recent studies have, however, shown that the energy levels can be changed by the mixing ratio of two molecules which have different permanent quadrupole moments. From the good correlation between the magnitude of the mixed film's energy shift and the constituent molecules’ permanent quadrupole moment, it was noted that the molecular quadrupole plays an important role in the energy shift. In this study, ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoemission spectroscopy (LEIPS) are applied to the mixed films of zinc phthalocyanine (ZnPc) and perfluorinated ZnPc (${\mathrm{F}}_{16}\mathrm{ZnPc}$), which have permanent quadrupole moments with opposite directions. From the precisely determined ionization energies and electron affinities, we directly determine the electronic polarization energy $D$ and electrostatic energy $S$ as a function of mixing ratio. Furthermore, we examined the molecular orientation dependence of $S$ and $D$ values. $D$ is almost independent of the mixing ratio (the difference is less than 0.2 eV over the range of mixing ratio) whereas $S$ differs by as much as 1.6 eV. The result clearly shows that the energy levels’ continuous shift by the mixing ratio originates in the electrostatic interaction, whose leading term is the charge-permanent quadrupole interaction.

We investigate the joint effect of viscous and Ohmic dissipation on electric current flow through a slit in a barrier dividing a graphene sheet in two. In the case of the no-slip boundary condition, we find that the competition between the viscous and Ohmic types of the charge flow results in the evolution of the current density profile from a concave to convex shape. We provide a detailed analysis of the evolution and identify favorable conditions to observe it in experiment. In contrast, in the case of the no-stress boundary condition, there is no qualitative difference between the current profiles in the Ohmic and viscous limits. The dichotomy between the behavior corresponding to distinct boundary conditions could be tested experimentally.

Fluctuations are significant in mesoscopic systems and of particular importance in understanding quantum transport. Here, we show that fluctuations can be considered as a resource for the operations of open quantum systems as functional devices. We derive the statistics of the thermal transistor amplification factor and the cooling-by-heating refrigerator efficiency under the Gaussian fluctuation framework. Statistical properties of the stochastic thermal transistor and the cooling-by-heating efficiency are revealed in the linear-response regime. We clarify the unique role of inelastic processes on thermal transport in mesoscopic systems. We further show that elastic and inelastic processes lead to different bounds based on the linear transport coefficients by using a generic theoretical framework for mesoscopic heat transport, which treats electron and bosonic collective excitations in an equal-footing manner. The underlying physics are illustrated concretely using a double-quantum-dot three-terminal system, though the theory applies to more general systems.

Shuttling of single electrons in gate-defined silicon quantum dots is numerically simulated. A minimal gate geometry without explicit tunnel barrier gates is introduced, and used to define a chain of accumulation mode quantum dots, each controlled by a single gate voltage. One-dimensional potentials are derived from a three-dimensional electrostatic model, and used to construct an effective Hamiltonian for efficient simulation. Control pulse sequences are designed by maintaining a fixed adiabaticity, so that different shuttling conditions can be systematically compared. We first use these tools to optimize the device geometry for maximum transport velocity, considering only orbital states and neglecting valley and spin degrees of freedom. Taking realistic geometrical constraints into account, charge shuttling speeds up to $\sim 300$ m/s preserve adiabaticity. Coherent spin transport is simulated by including spin-orbit and valley terms in an effective Hamiltonian, shuttling one member of a singlet pair and tracking the entanglement fidelity. With realistic device and material parameters, shuttle speeds in the range $10–100$ m/s with high spin entanglement fidelities are obtained when the tunneling energy exceeds the Zeeman energy. High fidelity also requires the interdot valley phase difference to be below a threshold determined by the ratio of tunneling and Zeeman energies, so that spin-valley-orbit mixing is weak. In this regime, we find that the primary source of infidelity is a coherent spin rotation that is correctable, in principle. The results pertain to proposals for large-scale spin qubit processors in isotopically purified silicon that rely on coherent shuttling of spins to rapidly distribute quantum information between computational nodes.

The boundary of one-dimensional topological superconductors might lead to the appearance of Majorana zero modes, whose nontrivial exchange statistics can be used for quantum computing. In branched nanowire networks, one can exchange Majorana modes by time dependently tuning topologically nontrivial parameter regions. In this paper, we simulate the exchange of four Majorana modes in T-shaped junctions made out of $p$-wave superconducting Rashba wires. We derive concrete experimental predictions for (quasi-)adiabatic braiding times and determine geometric conditions for successful Majorana exchange processes. Moreover, we prove that, in the adiabatic limit, the gating time needs to be smaller than the inverse of the squared superconducting order parameter and scales linearly with the gating potential. Furthermore, we show how to circumvent the formation of additional Majorana modes in branched nanowire systems, arising at wire intersection points of narrow junctions. Finally, we propose a multiqubit setup, allowing for universal quantum computing.

We apply a worm algorithm to simulate the quantum transverse-field Ising model in a path-integral representation of which the expansion basis is taken as the spin component along the external-field direction. In such a representation, a configuration can be regarded as a set of nonintersecting loops constructed by “kinks” for pairwise interactions and spin-down (or -up) imaginary-time segments. The wrapping probability for spin-down loops, a dimensionless quantity characterizing the loop topology on a torus, is observed to exhibit small finite-size corrections and yields a high-precision critical point in two dimensions (2D) as $h_c=3.044330\left(6\right)$, significantly improving over the existing results and nearly excluding the central value of the previous result $h_c=3.04438\left(2\right)$. At criticality, the fractal dimensions of the loops are estimated as $d_{\ell \downarrow }\left(1\mathrm{D}\right)=1.37\left(1\right)\approx \frac{11}{8}$ and $d_{\ell \downarrow }\left(2\mathrm{D}\right)=1.75\left(3\right)$, consistent with those for the classical 2D and 3D O(1) loop model, respectively. An interesting feature is that in one dimension (1D), both the spin-down and -up loops display the critical behavior in the whole disordered phase ($0\le h<h_c$), having a fractal dimension $d_{\ell }=1.750\left(7\right)$ that is consistent with the hull dimension $d_{\mathrm{H}}=\frac{7}{4}$ for critical 2D percolation clusters. The current worm algorithm can be applied to simulate other quantum systems like hard-core boson models with pairing interactions.

We approach the long-standing problem of preparing an out-of-equilibrium many-body Chern insulator (CI) and associated bulk-boundary correspondence unitarily. Herein, this is addressed by constructing a dynamical many-body Chern invariant exploiting the property of the bulk macroscopic electric polarization (Resta polarization) of the CI. This Chern invariant defined from observable correlations is also established to topologically classify many-body Chern states in equilibrium. The nonequilibrium behavior of the invariant is probed by ramping the paradigmatic Haldane model of graphene from its trivial to the topological phase. We show that a nonlinear ramp may work more efficiently in approaching the topological state, thereby establishing the existence of an optimal topological state preparation. Furthermore, to ensure the near adiabatic dynamics across the quantum critical point, we propose a counterdiabatic scheme. The topological nature of the prepared state is firmly established by observing an emerging $U\left(1\right)$ topological charge. We also compute the edge current in the time-evolved state of the system under a semiperiodic boundary condition, and we clearly establish an adiabatic bulk-boundary correspondence that firmly ensconces the validity of the many-body invariant.

The tetragonal ferrimagnetic ${\mathrm{Mn}}_3\mathrm{Ga}$ exhibits a wide range of intriguing magnetic properties. Here, we report the emergence of topologically nontrivial nodal lines in the absence of spin-orbit coupling (SOC) which are protected by both mirror and $C_{4z}$-rotational symmetries. In the presence of SOC, we demonstrate that the doubly degenerate nontrivial crossing points evolve into $C_{4z}$-protected Weyl nodes with chiral charge of $\pm 2$. Furthermore, we have considered the experimentally reported noncollinear ferrimagnetic structure where the magnetic moment of the ${\mathrm{Mn}}_I$ atom (on the Mn-Ga plane) is tilted by an angle $\theta$ with respect to the crystallographic $c$ axis. The evolution of the Weyl nodes with $\theta$ reveals that the double Weyl nodes split into a pair of charge-1 Weyl nodes whose separation can be tuned by the magnetic orientation in the noncollinear ferrimagnetic structure.

We study the pairing symmetry of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ through the group-theoretical approach. We emphasize the role of pairing interaction between the quasi-one-dimensional (Q1D) $d_{xz/yz}$ and quasi-two-dimensional (Q2D) $d_{xy}$ orbitals. It is found that two degenerate interorbital time-reversal-invariant(TRI) $p$-wave pairings, one spin singlet and the other spin triplet with out-of-plane $\mathit{d}$ vector, could be the most promising candidates. Several important physical quantities are presented, including the near-nodal gap structure, the unchanged out-of-plane Knight shift, and no splitting of transition under strain, which are consistent with the experiments. In addition, these $p$-wave pairings shed light on resolving the contradiction between the time-reversal breaking and reduced in-plane Knight shift measurements. As the system reaches the Van Hove singularity under applied strain, the pairing symmetry would become a $d$-wave pairing mainly consisting of interorbital components, which could be responsible for the strained $3K$ phase.

We study the stability of Kitaev quasiparticles in the presence of a perturbing Heisenberg interaction as a Fock space localization phenomenon. We identify parameter regimes where Kitaev states are localized, fractal, or delocalized in the Fock space of exact eigenstates, with the first two implying quasiparticle stability. Finite-temperature calculations show that a vison gap, and a nonzero plaquette Wilson loop at low temperatures, both characteristic of the deconfined Kitaev spin-liquid phase, persist far into the neighboring phase that has a concomitant stripy spin-density wave (SDW) order. The key experimental implication for Kitaev materials is that below a characteristic energy scale, unrelated to the SDW ordering, Kitaev quasiparticles are stable.

Some interacting disordered many-body systems are unable to thermalize when the quenched disorder becomes larger than a threshold value. Although several properties of nonzero energy density eigenstates (in the middle of the many-body spectrum) exhibit a qualitative change across this many-body localization (MBL) transition, many of the commonly used diagnostics only do so over a broad transition regime. Here we provide evidence that the transition can be located precisely even at modest system sizes by sharply defined changes in the distribution of extremal eigenvalues of the reduced density matrix of subsystems. In particular, our results suggest that $p*={lim}_{{\lambda }_2\to ln{\left(2\right)}^{+}}{P}_2\left({\lambda }_2\right)$, where $P_2\left({\lambda }_2\right)$ is the probability distribution of the second lowest entanglement eigenvalue ${\lambda }_2$, behaves as an “order parameter” for the MBL phase: $p*>0$ in the MBL phase, while $p*=0$ in the ergodic phase with thermalization. Thus, in the MBL phase, there is a nonzero probability that a subsystem is entangled with the rest of the system only via the entanglement of one subsystem qubit with degrees of freedom outside the region. In contrast, this probability vanishes in the thermal phase.

Strain-free ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$ thin films were obtained on MgO (001) single-crystal substrates with Cr seed layers. X-ray diffraction and transmission electron microscopy confirmed that the film is textured in all three spatial directions with crystallographic orientation determined by epitaxial constraints from the substrate. Average grain size was estimated to be in the range of tens of nanometers. Ten pieces of 40-nm-thick ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$ thin-film samples were stacked together for half-polarized neutron diffraction study at room temperature. A total of six reflections were usable, from which the magnetic structure of ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$ was determined and compared to existing models. Contrary to most electronic structure calculations, the present results suggest that the magnetic moment of Fe atoms at the $8h$ site is large and larger than that of the $4d$ site. Our results support the giant magnetization of ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$.

We present micromagnetic simulations of the dynamic GHz-range resonance modes of skyrmions excited by either out-of-plane AC magnetic fields or spin torques in prototypical synthetic ferri- and antiferromagnetic trilayer structures. The observed features in the calculated power spectra exhibit a systematic dependence on the coupling strength between the individual magnetic layers and are related to pure in-phase and antiphase breathing modes as well as to hybridizations of breathing and spin-wave modes that are characteristic for the considered circular-shaped geometry. The experimental detection of these resonant oscillation modes may provide a means for skyrmion-sensing applications and for the general characterization of skyrmion states in multilayer stacks with antiferromagnetic interlayer exchange coupling.

Recently $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ was predicted to be a magnetic Weyl semimetal with a lone pair of Weyl nodes generated by $A$-type antiferromagnetism and protected by a rotational symmetry. However, it was soon discovered that the actual magnetic structure broke the rotational symmetry and internal pressure was later suggested as a route to stabilize the desired magnetic state. In this work we test this prediction by synthesizing a series of ${\mathrm{Eu}}_{1-x}{\mathrm{Ba}}_x{\mathrm{Cd}}_2{\mathrm{As}}_2$ single crystals and studying their structural, magnetic, and transport properties via both experimental techniques and first-principles calculations. We find that small concentrations of Ba ($\sim 3\%–10\%$) lead to a small out-of-plane canting of the Eu moment. However, for higher concentrations this effect is suppressed and a nearly in-plane model is recovered. Studying the transport properties we find that all compositions show evidence of an anomalous Hall effect dominated by the intrinsic mechanism as well as large negative magnetoresistances in the longitudinal channel. A nonmonotonic evolution of the transport properties is seen across the series which correlates to the proposed canting suggesting canting may enhance the topological effects. Careful density functional theory calculations using an all-electron approach revise prior predictions finding a purely ferromagnetic ground state with in-plane moments for both the $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{As}}_2$ and ${\mathrm{Eu}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{Cd}}_2{\mathrm{As}}_2$ compounds, corroborating our experimental findings. This work suggests that Ba substitution can tune the magnetic properties in unexpected ways which correlate to changes in measures of topological properties, encouraging future work to locate the ideal Ba concentration for Eu moment canting.

In the study of three-dimensional gapped models, two-dimensional gapped states should be considered as a free resource. This is the basic idea underlying the notion of “foliated fracton order” proposed in Shirley et al. [Phys. Rev. X 8, 031051 (2018)]. We have found that many of the known type-I fracton models, although they appear very different, have the same foliated fracton order, known as “X-cube” order. In this paper, we identify three-dimensional fracton models with different kinds of foliated fracton order. Whereas the X-cube order corresponds to the gauge theory of a simple paramagnet with subsystem planar symmetry, the different orders correspond to twisted versions of the gauge theory for which the system prior to gauging has nontrivial order protected by the planar subsystem symmetry. We present constructions of the twisted models and demonstrate that they possess nontrivial order by studying their fractional excitation contents.

Higher order topological tnsulators are $d$-spatial dimensional systems featuring topologically protected gapless states at their $(d-n)$-dimensional boundaries. With the help of ab initio calculations and tight-binding models along with symmetry considerations, we show that monolayer buckled honeycomb structures of group-V elements (Sb, As), which have already been synthesized, belong in this category and have a spinless charge fractionalization of $\frac{e}{2}$ at the corner states as well as weak topological edge states, protected by the $S_6$ symmetry operation, which classify this system as a quadrupole topological insulator. The robustness of these edge and corner states to perturbations is explicitly demonstrated.