The two-dimensional Heisenberg exchange model with out-of-plane anisotropy and a Dzyaloshinskii-Moriya interaction is employed to investigate the lifetime and stability of antiferromagnetic (AFM) skyrmions as a function of temperature and external magnetic field. An isolated AFM skyrmion is metastable at zero temperature in a certain parameter range set by two boundaries separating the skyrmion state from the uniform AFM phase and a stripe domain phase. The distribution of the energy barriers for the AFM skyrmion decay into the uniform AFM state complements the zero-temperature stability diagram and demonstrates that the skyrmion stability region is significantly narrowed at finite temperatures. We show that the AFM skyrmion stability can be enhanced by an application of magnetic field, whose strength is comparable to the spin-flop field. This stabilization of AFM skyrmions in external magnetic fields is in sharp contrast to the behavior of their ferromagnetic counterparts. Furthermore, we demonstrate that the AFM skyrmions are stable on timescales of milliseconds below 50 K for realistic material parameters, making it feasible to observe them in modern experiments.

Identifying the electron dynamics in CuO chains has been elusive in Y-Ba-Cu-O (YBCO) cuprate systems when using standard angle-resolved photoemission spectroscopy (ARPES); a cleaved sample exhibits areas terminated by both CuO chains or BaO layers, and the size of a typical beam results in ARPES signals that are superposed from both terminations. Here, we employ spatially resolved ARPES with a submicrometric beam (nano-ARPES) to reveal the surface-termination-dependent electronic structures of the double CuO chains in ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$. We present an observation of sharp metallic dispersions and Fermi surfaces of the double CuO chains buried underneath the ${\mathrm{CuO}}_2$-plane block on a BaO-terminated surface. While the observed Fermi surfaces of the CuO chains are highly one dimensional, the electrons in the CuO chains do not undergo significant electron correlations and no signature of a Tomonaga-Luttinger liquid or a marginal Fermi liquid is found. Our work represents an important experimental step toward understanding the charge dynamics and provides a starting basis for modeling the high-$T_c$ superconductivity in YBCO cuprate systems.

We discuss the role of strong Coulomb interactions in iron-based superconductors (FeSCs). The presumed $s^{\pm }$ character of these superconductors means that the condensate is not symmetry protected against Coulomb repulsion. Remarkably, the transition temperatures and the excitation gap are quite robust across the large family of iron-based superconductors, despite drastic changes in Fermi-surface geometry. The Coulomb problem is to understand how these superconductors avoid the strong on-site Coulomb interaction of the iron atoms, while maintaining a robust transition temperature. Within the dominant space of $t_{2g}$ orbitals, on-site repulsion in the FeSCs forces two linearly independent components of the condensate to vanish. This raises the possibility that iron-based superconductors might adapt their condensate to the Coulomb constraints by rotating the pairing state within the large manifold of entangled extended $s$-wave gap functions with different orbital and momentum space structure. We examine this “orbital and k-space flexibility” (OKF) mechanism using both Landau theory and microscopic calculations within a multiorbital $t\text{−}J$ model. Based on our results, we conclude that OKF necessitates a large condensate degeneracy. One interesting possibility raised by our results is that a resolution to the Coulomb problem in FeSCs might require a reconsideration of triplet pairing.

With substantial evidence of glassy behavior in the phase diagram of high-$T_c$ superconductors and its coexistence with superconductivity, we attempt to answer the following question: what are the properties of a superconducting state where the force driving Cooper pairing becomes dissipative? We find that when the bosonic mediator is local, dissipation acts to reduce the superconducting critical temperature $\left(T_c\right)$. On the other hand, contrary to naïve expectations, $T_c$ behaves nonmonotonically with dissipation for a nonlocal mediator—weakly dissipative bosons at different energy scales act coherently to give rise to an increase in $T_c$ and eventually destroy superconductivity when the dissipation exceeds a critical value. The critical value occurs when dissipative effects become comparable to the energy scale associated with the spatial stiffness of the mediator, at which point $T_c$ acquires a maximum. We outline consequences of our results to recent proton-irradiation experiments (M. Leroux et al., arXiv:1808.05984) on the cuprate superconductor ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ (LBCO), which observe a disorder-induced increase in $T_c$ even when the transition temperature of the proximate charge density wave (CDW) seems to be unaffected by irradiation. Our mechanism is a way to raise $T_c$ that does not require a “tug-of-war” type scenario between two competing phases.

Topological pumping of edge states in finite crystals or quasicrystals with nontrivial topological phases provides a powerful means for robust excitation transfer. In most schemes of topological pumping, the edge states become delocalized and immersed into the continuum during the adiabatic cycle, requiring extremely slow evolution to avoid nonadiabatic effects. Here, a scheme of topological pumping based on adiabatic passage of edge and interface states is proposed, which is more robust to nonadiabatic effects and avoids delocalization of the state over the entire adiabatic cycle. The scheme is illustrated by considering adiabatic passage in a system sustaining two topologically protected edge states and one interface state, realized by interfacing two dimerized Su-Schrieffer-Heeger chains with different topological order.

Using Schwinger-boson mean-field theory, we calculate the dynamic spin structure factor at low temperatures $0<T\ll J$ for the spin-$1/2$ antiferromagnetic Heisenberg kagome model, within the gapped ${\mathbb{Z}}_2$ spin liquid phase Ansatz. We find that the spectral gap rapidly fills with temperature, with robust low-energy spectral weight developing by a temperature of $\mathrm{\Delta }/3$, where the spin gap is $2\mathrm{\Delta }$ (i.e., $\mathrm{\Delta }$ is the spinon gap), before any appreciable rise in spinon density or change in zero-temperature mean-field parameters. This is due to deconfinement of spinons which leads to terms suppressed only by $exp(-\mathrm{\Delta }/T)$. At still higher temperatures, the spinon density increases rapidly leading to a breakdown of the Schwinger-boson mean-field approach. We suggest that if the impurity-free spectral functions can be obtained through neutron scattering experiments on kagome herbertsmithites, temperature dependence of the subgap weight can provide distinct signatures of a ${\mathbb{Z}}_2$ quantum spin liquid.

Nitrogen-vacancy (NV) centers in diamond have attracted considerable interest in sensing of weak magnetic fields, such as those created by biological systems. Detecting such feeble signals requires near-surface NV centers, to reduce the distance between NVs and sources. Moreover, dense ensembles of NVs are highly desirable to reduce measurement time. However, robust charge-state switching is often observed in these systems, resulting in a complex interplay between charge and spin dynamics that can reduce the attainable level of spin polarization, and consequently, sensitivity. Understanding the mechanisms behind charge-state switching is, therefore, crucial to developing NV-based sensors. Here, we demonstrate a method to selectively measure charge dynamics in an ensemble of NVs by quenching the spin polarization using an off-axis magnetic field. Utilizing this technique, we show that, in nanodiamonds, charge-state instability increases with increasing NV density. In the case of bulk single crystal diamond, we show that NV centers located near the surface are more stable in the neutral $\left({\mathrm{NV}}^0\right)$ charge state, while the negatively charged $\left({\mathrm{NV}}^{-}\right)$ form is more stable in bulk.

A quantum mechanical approach and local response theory are applied to study plasmons propagating in nanometer-thin gold slabs sandwiched between different dielectrics. The metal slab supports two different kinds of modes, classified as long- and short-range plasmons. Quantum spill-out is found to significantly increase the imaginary part of their mode indices, and, surprisingly, even for slabs wide enough to approach bulk the increase is 20%. This is explained in terms of enhanced plasmonic absorption, which mainly takes place in narrow regions located near the slab surface.

We study the electronic structure of full-shell superconductor-semiconductor nanowires, which have recently been proposed for creating Majorana zero modes, using an eight-band $\vec{k}·\vec{p}$ model within a fully self-consistent Schrödinger-Poisson scheme. We find that the spin-orbit coupling induced by the intrinsic radial electric field is generically weak for subbands with their minimum near the Fermi energy. Furthermore, we show that the chemical potential windows consistent with the emergence of a topological phase are small and sparse and can only be reached by fine-tuning the diameter of the wire. These findings suggest that the parameter space consistent with the realization of a topological phase in full-shell InAs/Al nanowires is, at best, very narrow.

Monolayer $2H\text{−}{\mathrm{NbSe}}_2$ has recently been shown to be a two-dimensional superconductor, with a coexisting charge-density wave (CDW). As both phenomena are intimately related to electron-lattice interaction, a natural question is how superconductivity and CDW are interrelated through electron-phonon coupling (EPC), which is important to the understanding of two-dimensional superconductivity. This work investigates the superconductivity of monolayer ${\mathrm{NbSe}}_2$ in CDW phase using the anisotropic Migdal-Eliashberg formalism based on first-principles calculations. The mechanism of the competition between and coexistence of the superconductivity and CDW is studied in detail by analyzing EPC. It is found that the intrapocket scattering is related to superconductivity, leading to almost constant value of superconducting gaps on parts of the Fermi surface. The interpocket scattering is found to be responsible for CDW, leading to partial or full band gaps on the remaining Fermi surface. A recent experiment indicates that there is transitioning from regular superconductivity in thin-film ${\mathrm{NbSe}}_2$ to two-gap superconductivity in the bulk, which is shown here to have its origin in the extent of Fermi surface gapping of $\mathbf{K}$ and ${\mathbf{K}}^{\prime }$ pockets induced by CDW. Overall blue shifts of the phonons and sharp decrease of Eliashberg spectrum are found when the CDW forms.

We present a simple theory of thermoelectric transport in bilayer graphene and report our results for the electrical resistivity, the thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio as functions of doping density and temperature. In the absence of disorder, the thermal resistivity tends to zero as the charge neutrality point is approached; the electric resistivity jumps from zero to an intrinsic finite value, and the Seebeck coefficient diverges in the same limit. Even though these results are similar to those obtained for single-layer graphene, their derivation is considerably more delicate. The singularities are removed by the inclusion of a small amount of disorder, which leads to the appearance of a “window” of doping densities $0<n<n_c$ (with $n_c$ tending to zero in the zero-disorder limit) in which the Wiedemann-Franz law is severely violated.

Motivated by the realization of $\mathrm{SU}(N)$ antiferromagnetism with the multirow representations in cold-atom physics, we studied its low-energy nonlinear sigma model defined on the Grassmann manifold $\text{U}\left(N\right)/\text{U}\left(m\right)\text{U}(N-m)$ using the complex projective presentation, which is a direct generalization of the widely studied ${\mathrm{CP}}^{N-1}$ model (corresponding to $m=1$). Using the large-$N$ expansion up to the first order of $1/N$ with fixed $m$, and in space dimension $2<d<4$, we obtain the critical exponents, including in particular the gauge-independent exponents that are directly relevant for experimental measurements. These exponents are found to depend only on $m/N$, indicating that a larger $m$ effectively reduces $N$ and thus causes stronger fluctuations about the saddle point.

Quantum finite-frequency noise is one of the fundamental aspects in quantum measurements performed during quantum information processing where currently Majorana bound states offer an efficient way to implement fault-tolerant quantum computation via topological protection from decoherence or unitary errors. Thus, a detailed exploration of Majorana finite-frequency noise spectra, preferably in a nonequilibrium device, is a timely challenge of fundamental importance. Here, we present results on finite-frequency differential noise that is the derivative of the noise with respect to the frequency. This quantity has universal units of $e^2$ and scans in high detail all peculiarities of the Majorana noise, clearly demonstrating its universal finite-frequency features. In particular, we provide photon absorption spectra on all energy scales and reveal a rich structure including universal Majorana plateaus as well as universal Majorana resonances and antiresonances at characteristic frequencies. Our results are of immediate interest to state-of-the-art experiments involving quantum noise mesoscopic detectors able to separately measure photon absorption and emission spectra.

Recent progress in nanostructuring of materials opens up possibilities to achieve more efficient thermoelectric devices. Nanofilms, nanowires, and nanorings may show increased phonon scattering while keeping good electron transport, two of the basic ingredients for designing more efficient thermoelectric systems. Here we argue that graphene nanorings attached to two leads meet these two requirements. Using a density-functional parametrized tight-binding method combined with Green's function technique, we show that the lattice thermal conductance is largely reduced as compared to that of graphene nanoribbons. At the same time, numerical calculations based on the quantum transmission boundary method, combined with an effective transfer matrix method, predict that the electric properties are not considerably deteriorated, leading to an overall remarkable thermoelectric efficiency. We conclude that graphene nanorings can be regarded as promising candidates for nanoscale thermoelectric devices.

Radiative heat transfer (RHT) between two metallic nanoparticles clusters in both near field and far field are explored using many-body radiative heat transfer theory implemented with the coupled electric and magnetic dipole (CEMD) approach, which effectively takes into account the contribution of magnetic polarization of metallic nanoparticles on heat exchange. The effects of magnetic polarization, many-body interaction (MBI), fractal dimension, and relative orientation of the clusters on RHT were analyzed. The results show that the contribution of magnetically polarized eddy-current Joule dissipation dominates the RHT between Ag nanoparticle clusters. If the electric polarization (EP approach) only is considered, the heat conductance will be underestimated as compared with the CEMD approach in both near field and far field regime. The effect of MBI on the RHT between Ag nanoparticle clusters is insignificant at room temperature, which is quite different from the SiC nanoparticle clusters. For the latter, MBI tends to suppress RHT significantly. The relative orientation has remarkable effect on radiative heat flux for clusters with lacy structure when the separation distance is in the near field. While for the separation distance in far field, both the relative orientation and the fractal dimension has a weak influence on radiative heat flux. This work will help the understanding of thermal transport in dense particulate system.

MnGe has been reported as a candidate for a three-dimensional (3D) skyrmion crystal in comparison to the two-dimensional (2D) skyrmion observed in most other B20 compounds like MnSi. In addition, the small-sized skyrmions in MnGe are desired properties for information storage. By performing the density functional theory (DFT) calculations and model simulations based on the DFT-informed tight-binding Hamiltonian, we explore the nature of the 3D skyrmion in MnGe. By invoking the dual nature of $d$ electrons on Mn atoms, we propose a strong-correlation-derived spin-fermion model with an antiferromagnetic coupling between the localized and itinerant moments. This model could explain the drastic difference in magnetic moments between MnGe and MnSi compounds. In addition, we find that the 3D or 2D nature of skyrmions is dependent on the coupling strength.

Understanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and in-plane resistivity anisotropy in uniaxial pressure detwinned ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{SrFe}}_2{\mathrm{As}}_2$, the parent compounds of iron pnictide superconductors. While ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ exhibits weakly first-order tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase transitions below $T_s>T_N\approx 138\mathrm{K}, {\mathrm{SrFe}}_2{\mathrm{As}}_2$ has strongly coupled first-order structural and AF transitions below $T_s=T_N\approx 210\mathrm{K}$. We find that the direct signatures of the nematic phase persist to lower temperatures above the phase transition in the case of ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ compared to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Our findings support the conclusion that the strongly first-order nature of the magnetic transition in ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ weakens the nematic phase and resistivity anisotropy in the system.

We investigate the destructive inelastic as well as the elastic scattering of a hot electron-hole plasma with an incoherent exciton population in bulk Ge by means of optical pump-terahertz probe spectroscopy. An incoherent exciton population evolves from a first optical pulse while a delayed second optical pulse creates the electron-hole plasma. The interaction of the plasma with the exciton population is monitored via the intraexcitonic transitions by a probing terahertz pulse. Analyzing the density-dependent decay of the intraexcitonic transitions after the arrival of the second optical pulse yields an inelastic scattering rate of $2.0\times {10}^{-4}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. An analysis of the corresponding linewidth of the $1s-2p$ transition yields a total scattering rate of $3.7\times {10}^{-4}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. This allows us to experimentally distinguish between elastic and inelastic scattering and we obtain an elastic scattering rate of $1.7\times {10}^{-4}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$.

Realization of nonreciprocal devices, such as isolators and circulators, is of fundamental importance in microwave and photonic communication systems. This can be achieved by breaking time-reversal symmetry in the system or exploiting nonlinearity and topological effects. However, exploration of nonreciprocal devices remains scarce in acoustic systems. In this work, sound propagation in a space-time modulated medium is theoretically studied. Finite-difference time-domain simulations are carried out to verify the results. Functionalities such as mode conversion, parametric amplification, and phase conjugation are demonstrated.