We derive equations of motion for topological solitons in antiferromagnets under the combined action of perturbations such as an external magnetic field and torque-generating electrical current. Aside from conservative forces, such perturbations generate an effective “magnetic field” exerting a gyrotropic force on the soliton and an induced “electric field” if the perturbation is time-dependent. We apply the general formalism to the cases of a domain wall and of a vortex. An antiferromagnetic vortex can be effectively moved by combined applications of a magnetic field and an electric current.

We study the dynamics of a transverse-field $\mathrm{XY}$ chain driven across quantum critical points by noisy control fields. We characterize the defect density as a function of the quench time and the noise strength and demonstrate that the defect productions for three quench protocols with different scaling exponents exhibit the anti-Kibble-Zurek behavior, whereby slower driving results in more defects. The protocols are quenching through the boundary line between paramagnetic and ferromagnetic phases, quenching across the isolated multicritical point, and quenching along the gapless line, respectively. We also show that the optimal quench time to minimize defects scales as a universal power law of the noise strength in all three cases. Furthermore, by using quantum simulation of the quench dynamics in the spin system with well-designed Landau-Zener crossings in pseudomomentum space, we propose an experimentally feasible scheme to test the predicted anti-Kibble-Zurek behavior of this noisy transverse-field $\mathrm{XY}$ chain with two-level systems under controllable fluctuations.

Using soft x-ray absorption spectroscopy we determined the chemical and magnetic properties of the magnetic topological insulator (MTI) Cr:${\mathrm{Sb}}_2{\mathrm{Te}}_3$. X-ray magnetic circular dichroism (XMCD) at the Cr $L_{2,3}$, Te $M_{4,5}$, and Sb $M_{4,5}$ edges shows that the Te $5p$ moment is aligned antiparallel to both the Cr $3d$ and Sb $5p$ moments, which is characteristic for carrier-mediated ferromagnetic coupling. Comparison of the Cr $L_{2,3}$ spectra with multiplet calculations indicates a hybridized Cr state, consistent with the carrier-mediated coupling scenario. We studied the enhancement of the Curie temperature, $T_{\mathrm{C}}$, of the MTI thin film through the magnetic proximity effect. Arrott plots, measured using the Cr $L_3$ XMCD, show a $T_{\mathrm{C}}\approx 87$ K for the as-cleaved film. After deposition of a thin layer of ferromagnetic Co onto the surface, the $T_{\mathrm{C}}$ increases to $\sim 93$ K, while the Co and Cr moments are parallel. This increase in $T_{\mathrm{C}}$ is unexpectedly small compared to similar systems reported earlier. The XMCD spectra demonstrate that the Co/MTI interface remains intact, i.e., no reaction between Co and the MTI takes place. Our results are a useful starting point for refining the physical models of Cr-doped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, which is required for making use of them in device applications.

Refocalization sequences in nuclear magnetic resonance (NMR) can in principle reverse the coherent evolution under the secular dipolar Hamiltonian of a closed system. We use this experimental strategy to study the effect of irreversible decoherence on the signal amplitude attenuation in a single-crystal hydrated salt where the nuclear spin system consists of the set of hydration water proton spins having a strong coupling within each pair and a much weaker coupling with other pairs. We study the experimental response of attenuation times with temperature, crystal orientation with respect to the external magnetic field, and rf pulse amplitudes. We find that the observed attenuation of the refocalized signals can be explained by two independent mechanisms: (a) evolution under the nonsecular terms of the reversion Hamiltonian, and (b) an intrinsic mechanism having the attributes of irreversible decoherence induced by the coupling with a quantum environment. To characterize (a) we compare the experimental data with the numerical calculation of the refocalized NMR signal of an artificial, closed spin system. To describe (b) we use a model of the irreversible adiabatic decoherence of spin pairs coupled with a phonon bath which allows evaluating an upper bound for the decoherence times. This model accounts for both the observed dependence of the decoherence times on the eigenvalues of the spin-environment Hamiltonian, and the independence from the sample temperature. This result, then, supports the adiabatic decoherence induced by the dipole-phonon coupling as the explanation for the observed irreversible decay of reverted NMR signals in solids.

Persistent currents in Bose condensates with a scalar order parameter are stabilized by the topology of the order parameter manifold. In condensates with multicomponent order parameters it is topologically possible for supercurrents to “unwind” without leaving the manifold. We study the energetics of this process in the case of ferromagnetic condensates using a long wavelength energy functional that includes both the superfluid and spin stiffnesses. Exploiting analogies to an elastic rod and rigid body motion, we show that the current carrying state in a 1D ring geometry transitions between a spin helix in the energy minima and a solitonlike configuration at the maxima. The relevance to recent experiments in ultracold atoms is briefly discussed.

We propose a feasible and scalable quantum-dot-based implementation of a singlet-only spin qubit which is to leading order intrinsically insensitive to random effective magnetic fields set up by fluctuating nuclear spins in the host semiconductor. Our proposal thus removes an important obstacle for further improvement of spin qubits hosted in high-quality III-V semiconductors such as GaAs. We show how the resulting qubit could be initialized, manipulated, and read out by electrical means only, in a way very similar to a triple-dot exchange-only spin qubit. Due to the intrinsic elimination of the effective nuclear fields from the qubit Hamiltonian, we find an improvement of the dephasing time $T_2^{*}$ of several orders of magnitude as compared to similar existing spin qubits.

Exciton polaritons are a coherent electron-hole-photon (e-h-p) system where condensation has been observed in semiconductor microcavities. In contrast to equilibrium Bose-Einstein condensation (BECs) for long lifetime systems, polariton condensates have a dynamical nonequilibrium feature owing to the similar physical structure that they have to semiconductor lasers. One of the distinguishing features of a condensate to a laser is the presence of strong coupling between the matter and photon fields. Irrespective of its equilibrium or nonequilibrium nature, exciton polaritons have been observed to maintain strong coupling. We show that by investigating the high-density regime of exciton-polariton condensates, the negative branch is directly observed in photoluminescence. This is evidence that the present e-h-p system is still in the strong-coupling regime, contrary to past results where the system reduced to standard lasing at high density.

Many bosonic (fermionic) fractional quantum Hall states, such as Laughlin, Moore-Read, and Read-Rezayi wave functions, belong to a special class of orthogonal polynomials: the Jack polynomials (times a Vandermonde determinant). This fundamental observation allows one to point out two different recurrence relations for the coefficients of the permanent (Slater) decomposition of the bosonic (fermionic) states. Here we provide an explicit Fock space representation for these wave functions by introducing a two-body squeezing operator which represents them as a Jastrow operator applied to reference states, which are, in general, simple periodic one-dimensional patterns. Remarkably, this operator representation is the same for bosons and fermions, and the different nature of the two recurrence relations is an outcome of particle statistics.

We investigate the helically propagating edge states associated with pseudo-Landau levels in strained honeycomb lattices. We exploit chiral symmetry to derive a general criterion for the existence of these propagating edge states in the presence of only nearest-neighbor hoppings and we verify our criterion using numerical simulations of both uniaxially and trigonally strained honeycomb lattices. We show that the propagation of the helical edge state can be controlled by engineering the shape of the edges. Sensitivity to chiral-symmetry-breaking next-nearest-neighbor hoppings is assessed. Our result opens up an avenue toward the precise control of edge modes through manipulation of the edge shape.

A metal nanoparticle coupled to a semiconductor quantum dot forms a tunable hybrid system which exhibits remarkable optical phenomena. Small metal nanoparticles possess nanocavitylike optical concentration capabilities due to the presence of strong dipolar excitation modes in the form of localized surface plasmons. Semiconductor quantum dots have strong luminescent capabilities widely used in many applications such as biosensing. When a quantum dot is kept in the vicinity of a metal nanoparticle, a dipole-dipole coupling occurs between the two nanoparticles giving rise to various optical signatures in the scattered spectra. This coupling makes the two nanoparticles behave like a single hybrid molecule. Hybrid molecules made of metal nanoparticles (MNPs) and quantum dots (QDs) under the influence of an external driving field have been extensively studied in literature, using the local response approximation (LRA). However, such previous work in this area was not adequate to explain some experimental observations such as the size-dependent resonance shift of metal nanoparticles which becomes quite significant with decreasing diameter. The nonlocal response of metallic nanostructures which is hitherto disregarded by such studies is a main reason for such nonclassical effects. The generalized nonlocal optical response (GNOR) model provides a computationally less-demanding path to incorporate such properties into the theoretical models. It allows unified theoretical explanation of observed experimental phenomena which previously seemed to require ab initio microscopic theory. In this paper, we analyze the hybrid molecule in an external driving field as an open quantum system using a cavity-QED approach. In the process, we quantum mechanically model the dipole moment operator and the dipole response field of the metal nanoparticle taking the nonlocal effects into account. We observe that the spectra resulting from the GNOR based model effectively demonstrate the experimentally observed size dependent amplitude scaling, linewidth broadening, and resonance shift phenomena compared to the respective LRA counterparts. Then, we provide a comparison between our suggested GNOR based cavity-QED model and the conventional LRA model, where it becomes evident that our analytical model provides a close match to the experimentally suggested behavior. Furthermore, we show that the Rayleigh scattering spectra of the MNP-QD hybrid molecule possess an asymmetric Fano interference pattern that is tunable to suit various applications.

We demonstrate new mechanisms for gate-tunable current partition at topological zero-line intersections in a graphene-based current splitter. Based on numerical calculations of the nonequilibrium Green's functions and Landauer-Büttiker formula, we show that the presence of a perpendicular magnetic field on the order of a few Teslas allows for carrier sign dependent current routing. In the zero-field limit the control on current routing and partition can be achieved within a range of 10-$90\%$ of the total incoming current by tuning the carrier density at tilted intersections or by modifying the relative magnitude of the bulk band gaps via gate voltage. We discuss the implications of our findings in the design of topological zero-line networks where finite orbital magnetic moments are expected when the current partition is asymmetric.

We present theoretical and numerical investigations of vortices driven by strong dc and ac currents in long Josephson junctions described by a nonlinear integro-differential equation which takes into account nonlocal electrodynamics of films, vortex bremsstrahlung, and Cherenkov radiation amplified by the attraction of vortices to the edges of the junction. This work focuses on the dynamics of vortices in Josephson junctions in thin films where the effects of Josephson nonlocality are essential but London screening is negligible. We obtained an exact solution for a vortex driven by an arbitrary time-dependent current in an overdamped junction where the vortex turns into a phase slip if the length of the junction is shorter than a critical length which depends on current. Our analytical and numerical results show that the dynamic behavior of vortices depends crucially on the ohmic damping parameter. In overdamped junctions vortices expand as they move faster and turn into phase slips as current increases. In underdamped junctions vortices entering from the edges produce Cherenkov radiation generating cascades of expanding vortex-antivortex pairs, which ultimately drive the entire junction into a resistive phase slip state. Simulations revealed a variety of complex dynamic states of vortices under dc and ac currents which can manifest themselves in hysteretic current-voltage characteristics with jumps and regions with negative differential resistance resulting from transitions from oscillating to ballistic propagation of vortices, their interaction with pinning centers, and standing nonlinear waves in the junction.

Propagation character of spin wave was investigated for chiral magnets FeGe and Co-Zn-Mn alloys, which can host magnetic skyrmions near room temperature. On the basis of the frequency shift between counterpropagating spin waves, the magnitude and sign of Dzyaloshinskii-Moriya (DM) interaction were directly evaluated. The obtained magnetic parameters quantitatively account for the size and helicity of skyrmions as well as their materials variation, proving that the DM interaction plays a decisive role in the skyrmion formation in this class of room-temperature chiral magnets. The propagating spin-wave spectroscopy can thus be an efficient tool to study DM interaction in bulk single-phase compounds. Our results also demonstrate a function of spin-wave diode based on chiral crystal structures at room temperature.

${\mathrm{Nd}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$, belonging to the family of geometrically frustrated cubic rare-earth pyrochlore oxides, was recently identified to order antiferromagnetically below $T_{\mathrm{N}}\approx 0.55$ K with an all-in/all-out arrangement of ${\mathrm{Nd}}^{3+}$ moments, however, with a much reduced ordered state moment. Herein, we investigate the spin dynamics and crystal-field states of ${\mathrm{Nd}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ using muon spin relaxation $\left(\mu \mathrm{SR}\right)$ and inelastic neutron scattering (INS) measurements. Our $\mu \mathrm{SR}$ study confirms the long-range magnetic ordering and shows evidence for coexisting persistent dynamic spin fluctuations deep inside the ordered state down to 42 mK. The INS data show the crystal electric field (CEF) excitations due to the transitions both within the ground-state multiplet and to the first excited state multiplet. The INS data are analyzed by a model based on CEF and crystal-field states are determined. Strong Ising-type anisotropy is inferred from the ground-state wave function. The CEF parameters indicate the CEF-split Kramers doublet ground state of ${\mathrm{Nd}}^{3+}$ to be consistent with the dipolar-octupolar character.

We have systematically investigated the magnetic properties of ${\mathrm{La}}_{}$- and ${\mathrm{Co}}_{}$-substituted ${\mathrm{Sr}}_{}{\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ using single crystals. By utilizing the traveling solvent floating zone technique, we have grown single crystals over a wide range of substitution content, and examined their saturation moments and magnetocrystalline anisotropy. With increasing content of ${\mathrm{La}}_{}$ and ${\mathrm{Co}}_{}$, the saturation moment at $300\mathrm{K}$ monotonically increases. The increment of magnetocrystalline anisotropy is almost proportional to the content of Co. In addition, further La substitution also enhances magnetocrystalline anisotropy, which demonstrates that ${\mathrm{Fe}}^{2+}$ plays an important role to enhance magnetocrystalline anisotropy. We have analyzed magnetocrystalline anisotropy energy as a function of substitution content, and elucidate the effects of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Fe}}^{2+}$.

The classification of topological phases of matter in the presence of interactions is an area of intense interest. One possible means of classification is via studying the partition function under modular transforms, as the presence of an anomalous phase arising in the edge theory of a $D$-dimensional system under modular transformation, or modular anomaly, signals the presence of a $(D+1)$-D nontrivial bulk. In this work, we discuss the modular transformations of conformal field theories along a $(2+1)$-D and a $(3+1)$-D edge. Using both analytical and numerical methods, we show that chiral complex free fermions in $(2+1)$-D and $(3+1)$-D are modular invariant. However, we show in $(3+1)$-D that when the edge theory is coupled to a background $U\left(1\right)$ gauge field, this results in the presence of a modular anomaly that is the manifestation of a quantum Hall effect in a $(4+1)$-D bulk. Using the modular anomaly, we find that the edge theory of a $(4+1)$-D insulator with space-time inversion symmetry $\left(PT\right)$ and fermion number parity symmetry for each spin becomes modular invariant when eight copies of the edges exist.

Solid solution MAX phases offer the opportunity for further tuning of the thermomechanical and functional properties of MAX phases, increasing their envelope of performance. Previous experimental results show that the lattice parameters of ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ decrease, while the Young's modulus increases with increased Si content in the lattice. In this work, we present a computational investigation of the structural, electronic, and mechanical properties of ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ $(x=0$, 0.25, 0.5, 0.75, and 1). The solid solutions were modeled using special quasirandom structures (SQS) and calculated using density functional theory (DFT), which is implemented in the Vienna ab initio simulation package (VASP). The SQS structures represent random mixing of Al and Si in the A sublattice of 312 MAX phase and their structural, electronic, and mechanical properties were calculated and compared with experiments. We study the cleavage and slip behavior of ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ to investigate the deformation behavior in terms of cleavage and shear. It has been shown that the cleavage between M and A layers results in increasing cleavage stress in ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ as a function of Si content in the lattice. In addition, the shear deformation of hexagonal close packed ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ under $〈2\overline{1}\overline{1}0〉\left\{0001\right\}$ and $〈0\overline{1}10〉\left\{0001\right\}$ results in increasing unstable stacking fault energy (USFE) and ideal shear strength (ISS) in ${\mathrm{Ti}}_3\left({\mathrm{Si}}_x{\mathrm{Al}}_{1-x}\right){\mathrm{C}}_2$ as the system becomes richer in Si.

We report single-crystal growth and physical property characterization of ${\mathrm{YbFe}}_2{\mathrm{Al}}_{10}$ compounds. The measurements of resistivity, magnetic susceptibility, and specific heat show different behaviors from previous studies on polycrystalline samples. An intermediate-valence characteristic with moderate mass enhancement is indicated. In particular, the optical spectroscopy measurement reveals formation of multiple hybridization energy gaps which become progressively pronounced at low temperature. The multiple hybridization energy gaps are likely caused by the hybridizations between the flat band from Yb $4f$ electrons and different bands of conduction electrons.

Resorting to a recently developed theoretical device called dimensional regularization for quantum criticality with a Fermi surface, we examine a metal-insulator quantum phase transition from a Landau's Fermi-liquid state to a U(1) spin-liquid phase with a spinon Fermi surface in two dimensions. Unfortunately, we fail to approach the spin-liquid Mott quantum critical point from the U(1) spin-liquid state within the dimensional regularization technique. Self-interactions between charge fluctuations called holons are not screened, which shows a run-away renormalization group flow, interpreted as holons remain gapped. This leads us to consider another fixed point, where the spinon Fermi surface can be destabilized across the Mott transition. Based on this conjecture, we reveal the nature of the spin-liquid Mott quantum critical point: Dimensional reduction to one dimension occurs for spin dynamics described by spinons. As a result, Landau damping for both spin and charge dynamics disappear in the vicinity of the Mott quantum critical point. When the flavor number of holons is over its critical value, an interacting fixed point appears to be identified with an inverted $XY$ universality class, controlled within the dimensional regularization technique. On the other hand, a fluctuation-driven first order metal-insulator transition results when it is below the critical number. We propose that the destabilization of a spinon Fermi surface and the emergence of one-dimensional spin dynamics near the spin-liquid Mott quantum critical point can be checked out by spin susceptibility with a $2k_F$ transfer momentum, where $k_F$ is a Fermi momentum in the U(1) spin-liquid state: The absence of Landau damping in U(1) gauge fluctuations gives rise to a divergent behavior at zero temperature while it vanishes in the presence of a spinon Fermi surface.