Although extensive experimental and theoretical works have been conducted to understand the ballistic and diffusive phonon transport in nanomaterials recently, direct observation of temperature and thermal nonequilibrium of different phonon modes has not been realized. Herein, we have developed a method within the framework of molecular dynamics to calculate the temperatures of phonons in both real and phase spaces. Taking silicon thin film and graphene as examples, we directly obtained the spectral phonon temperature (SPT) and observed the local thermal nonequilibrium between the ballistic and diffusive phonons. Such nonequilibrium also generally exists across interfaces and is surprisingly large, and it provides a significant additional thermal interfacial resistance mechanism besides phonon reflection. Our SPT results directly show that the vertical thermal transport across the dimensionally mismatched graphene-substrate interface is through the coupling between flexural acoustic phonons of graphene and the longitudinal phonons in the substrate with mode conversion. In the dimensionally matched interfaces, e.g., graphene-graphene junction and graphene-boron nitride planar interfaces, strong coupling occurs between the acoustic phonon modes on both sides, and the coupling decreases with interfacial mixing. The SPT method together with the spectral heat flux can eliminate the size effect of the thermal conductivity prediction induced from ballistic transport.

Under the framework of Taylor series expansion for potential energy, we propose a simple and robust metric, dubbed “regular residual analysis,” to measure the fourth-order phonon anharmonicity in crystals. The method is verified by studying the intrinsic strong higher-order anharmonic effects in ${\mathrm{UO}}_2$ and ${\mathrm{CeO}}_2$. Comparison of the thermal conductivity results, which calculated by the anharmonic lattice dynamics method coupled with the Boltzmann transport equation and the spectral energy density method coupled with ab initio molecular dynamics simulation further validates our analysis. Analysis of the bulk Si and Ge systems confirms that the fourth-order phonon anharmonicity is enhanced and cannot be neglected at high enough temperatures, which agrees with a previous study where the four-phonon scattering was explicitly determined. This metric will facilitate evaluating and interpreting the lattice thermal conductivity of crystals with strong fourth-order phonon anharmonicity.

In semiconductor quantum dots, coupling to the environment, i.e., to phonons, plays a crucial role for optical state preparation. We analyze the phonon impact on two methods for direct optical preparation of the dark exciton, which is enabled by a tilted magnetic field: excitation with a chirped laser pulse and excitation with a detuned pulse. Our study reveals that for both methods, phonons either do not impede the proposed mechanism or they are made useful by widening the parameter range where dark state preparation is possible due to phonon-assisted dark exciton preparation. In view of the positive impact of phonons on optical preparation, the use of dark excitons in quantum dots becomes even more attractive.

We theoretically study the band structure and the electronic transport in the Weyl semimetal nanowires in magnetic fields, and demonstrate that the interplay of the Fermi-arc surface states and the bulk Landau levels plays a crucial role in the magnetotransport. We show that a magnetic field perpendicular to the surface immediately hybridizes the counterpropagating surface modes into a series of dispersionless zeroth Landau levels, and it leads to a significant reduction of the traveling modes and a rapid decay of the conductance. On the contrary, a magnetic field parallel to the wire adds linearly dispersed zeroth Landau levels to the traveling modes and increases the conductance.

The helical edge states in a quantum spin Hall insulator are presumably protected by time-reversal symmetry. However, even in the presence of magnetic field which breaks time-reversal symmetry, the helical edge conduction can still exist, dubbed as pseudo quantum spin Hall effect. In this paper, the effects of the magnetic fields on the pseudo quantum spin Hall effect and the phase transitions are studied. We show that an in-plane magnetic field drives a pseudo quantum spin Hall state to a metallic state at a high field. Moreover, at a fixed in-plane magnetic field, an increasing out-of-plane magnetic field leads to a reentrance of pseudo quantum spin Hall state in an inverted InAs/GaSb quantum well. The edge state probability distribution and Chern numbers are calculated to verify that the reentrant states are topologically nontrivial. The origin of the reentrant behavior is attributed to the nonmonotonic bending of Landau levels and the Landau level mixing caused by the orbital effect induced by the in-plane magnetic field. The robustness to disorder is demonstrated by the numerically calculated quantized conductance for disordered nanowires within Landauer-Büttiker formalism.

We have studied the metal-insulator (MI) transition of indium nanowires on the Si(111) surface by electrical conductivity measurements with a four-point probe. Upon cooling, the sheet conductivity of the high-temperature $(4\times 1)$ phase, which is known to have metallic electron bands, exhibited a gradual decrease in quantitative agreement with the Mott's variable range hopping conduction. Upon further cooling, the conductivity exhibited a sharp drop at 120 K, indicating the transition into the insulating $(8\times 2)$ phase. The conductivity upon heating from 65 K did not trace the curve during cooling but showed a thermal hysteresis with a width of 8 K. The observation of the hysteresis agrees with the previous electron diffraction experiments, showing that the MI transition is first order. It was further found that, instead of the superheating behavior usually observed in first-order transitions, the transition upon heating starts below $T_c$, while the supercooling is always observed. This indicates a specific heterogeneous nucleation process only during heating. It is suggested that this is due to the destabilization of the nanowires near the domain boundaries. This is corroborated by the significant decrease of the transition temperature observed on a substrate with a high step density.

Understanding Luttinger holes in low dimensions is crucial for numerous spin-dependent phenomena and nanotechnology. In particular, hole quantum wires that are proximity coupled to a superconductor is a promising system for the observation of Majorana fermions. Earlier treatments of confined Luttinger holes ignored a mutual transformation of heavy and light holes at the heteroboundaries. We derive the effective hole Hamiltonian in the ground state. The mutual transformation of holes is crucial for Zeeman and spin-orbit coupling, and results in several spin-orbit terms linear in momentum in hole quantum wires. We discuss the criterion for realizing Majorana modes in charge carrier hole systems. GaAs or InSb hole wires shall exhibit stronger topological superconducting pairing, and provide additional opportunities for its control compared to InSb electron systems.

Massless Dirac fermions in graphene provide unprecedented opportunities to realize the Klein paradox, which is one of the most exotic and striking properties of relativistic particles. In the seminal theoretical work [M. I. Katsnelson et al., Nat. Phys. 2, 620 (2006)], it was predicted that the massless Dirac fermions can pass through one-dimensional (1D) potential barriers unimpededly at normal incidence. Such a result seems to preclude confinement of the massless Dirac fermions in graphene by using 1D potential barriers. Here, we demonstrate both experimentally and theoretically that massless Dirac fermions can be trapped in a quasi-1D $npn$ junction of a continuous graphene monolayer. Because of highly anisotropic transmission of the massless Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in graphene), charge carriers incident at large oblique angles will be reflected from one edge of the junction with high probability and continue to bounce from the opposite edge. Consequently, these electrons are trapped for a finite time to form quasibound states in the quasi-1D $npn$ junction. The quasibound states seen as pronounced resonances are probed and the quantum interference patterns arising from these states are directly visualized in our scanning tunneling microscope measurements.

We show that the bulk winding number characterizing one-dimensional topological insulators with chiral symmetry can be detected from the displacement of a single particle, observed via losses. Losses represent the effect of repeated weak measurements on one sublattice only, which interrupt the dynamics periodically. When these do not detect the particle, they realize negative measurements. Our repeated measurement scheme covers both time-independent and periodically driven (Floquet) topological insulators, with or without spatial disorder. In the limit of rapidly repeated, vanishingly weak measurements, our scheme describes non-Hermitian Hamiltonians, as the lossy Su-Schrieffer-Heeger model of Rudner and Levitov, [Phys. Rev. Lett. 102, 065703 (2009)]. We find, contrary to intuition, that the time needed to detect the winding number can be made shorter by decreasing the efficiency of the measurement. We illustrate our results on a discrete-time quantum walk, and propose ways of testing them experimentally.

In this paper, we consider a layered heterostructure of an Abelian topologically ordered state (TO), such as a fractional Chern insulator (FCI)/quantum Hall state (FQH) with an $s$-wave superconductor in order to explore the existence of non-Abelian defects. To uncover such defects, we note that the ground state corresponds to a charge $2e$ Cooper pair, the electron can no longer be treated as a local particle, and hence we must consider a larger TO due to the presence of $h/2e$ flux vortices, which strictly speaking are not deconfined. Quantum dimension and species of the defects follow directly from the fusion algebra. For FCI/Laughlin states, we show the presence of three kinds of defects, two of which had been previously ignored. They owe their origin to a general anyon permutation symmetry (AS) that exists in any fermionic Abelian TO state in contact with an $s$-wave superconductor. Physically, this permutation corresponds to adding a fermion to odd flux vortices (in units of $h/2e)$ as they travel around the associated topological (twist) defect. As such, we call it a fermion parity flip AS. We show that calculations can be handled more simply, by considering an equivalent fermion parity gauged theory, where the original TO is suitably augmented by a ${\mathbb{Z}}_2$ gauge sector coming from the $s$-wave SC, but with identical fusion structure. This trick makes our approach useful for analyzing a wide variety of FQH/FCI heterostructures. We give examples of the fermion parity gauging procedure for a large number of hierarchy and spin singlet states. We consider twist defects which mutate anyons according to the fermion parity flip symmetry and show that they can be realized at domain walls between distinct gapped edges or interfaces of the TO superconducting state. We analyze the properties of such defects and show that fermion parity flip twist defects are always associated with Majorana zero modes. When defects corresponding to AS which is a combination of fermion parity flip and charge conjugation are considered, they lead to ${\mathbb{Z}}_{2n+1}$ parafermions in Laughlin $1/(2n+1)$ states. Our formalism also reproduces known results such as Majorana/parafermionic bound states at superconducting domain walls of topological/fractional Chern insulators when twist defects are constructed based on charge conjugation symmetry. Finally, we briefly describe more exotic twist liquid phases obtained by gauging the AS where the twist defects become deconfined anyonic excitations.

We study the non-Gaussian dynamics of a quasiclassical electronic circuit coupled to a mesoscopic conductor. Non-Gaussian noise accompanying the nonequilibrium transport through the conductor significantly modifies the stationary probability density function (PDF) of the flux in the dissipative circuit. We incorporate weak quantum fluctuation of the dissipative LC circuit with a stochastic method and evaluate the quantum correction of the stationary PDF. Furthermore, an inverse formula to infer the statistical properties of the non-Gaussian noise from the stationary PDF is derived in the classical-quantum crossover regime. The quantum correction is indispensable to correctly estimate the microscopic transfer events in the QPC with the quasiclassical inverse formula.

We propose a Hamiltonian dynamics formalism for the current and magnetic field driven dynamics of ferromagnetic and antiferromagnetic domain walls in one-dimensional systems. To demonstrate the power of this formalism, we derive Hamilton equations of motion via Poisson brackets based on the Landau-Lifshitz-Gilbert phenomenology, and add dissipative dynamics via the evolution of the energy. We use this approach to study current induced domain-wall motion and compute the drift velocity. For the antiferromagnetic case, we show that a nonzero magnetic moment is induced in the domain wall, which indicates that an additional application of a magnetic field would influence the antiferromagnetic domain-wall dynamics. We consider both cases of the magnetic field being parallel and transverse to the Néel field. Based on this formalism, we predict an orientation switch mechanism for antiferromagnetic domain walls which can be tested with the recently discovered Néel spin orbit torques.

The layered perovskites $R\mathrm{Cr}$(${\mathrm{BO}}_3$)${}_2$ ($R=\mathrm{Y}$ and Ho) with magnetic triangular lattices were studied by performing ac/dc susceptibility, specific heat, elastic and inelastic neutron scattering, and dielectric constant measurements. The results show (i) both samples' ${\mathrm{Cr}}^{3+}$ spins order in a canted antiferromagnetic structure with $T_N$ around 8–9 K, while the ${\mathrm{Ho}}^{3+}$ ions do not order down to $T=1.5 \mathrm{K}$ in $\mathrm{HoCr}({\mathrm{BO}}_3$)${}_2$; (ii) when a critical magnetic field $H_C$ around 2-3 T is applied below $T_N$, the ${\mathrm{Cr}}^{3+}$ spins in the Y compound and both the ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ spins in the Ho compound order in a ferromagnetic state; (iii) both samples exhibit dielectric constant anomalies around the transition temperature and critical field, but the Ho compound displays a much stronger magnetodielectric response. We speculate that this is due to the magnetostriction, which depends on both the ${\mathrm{Cr}}^{3+}$ and the ${\mathrm{Ho}}^{3+}$ ions' ordering in the Ho compound. Moreover, by using linear spin-wave theory to simulate the inelastic neutron scattering data, we estimated the Y compound's intralayer and interlayer exchange strengths as ferromagnetic ${\mathrm{J}}_1=–0.12 \mathrm{meV}$ and antiferromagnetic ${\mathrm{J}}_2=0.014 \mathrm{meV}$, respectively. The competition between different kinds of superexchange interactions results in the ferromagnetic intralayer interaction.

We report measurements of the frequency and temperature dependence of ferromagnetic resonance (FMR) for a 15-nm-thick yttrium iron garnet (YIG) film grown by off-axis sputtering. Although the FMR linewidth is narrow at room temperature [corresponding to a damping coefficient $\alpha =(9.0\pm 0.2)\times {10}^{-4}]$, comparable to previous results for high-quality YIG films of similar thickness, the linewidth increases strongly at low temperatures, by a factor of almost 30. This increase cannot be explained as due to two-magnon scattering from defects at the sample interfaces. We point out that the increased low-temperature linewidth can be explained by impurity relaxation mechanisms that were elucidated 50 years ago in bulk YIG samples. High-purity starting materials and careful optimization of growth protocols to avoid nonstoichiometries should therefore be employed for making low-temperature thin-film YIG devices.

Inelastic electron scattering is a consequence of mostly Coulomb interaction between electrons in the sample and electron beam and, as such, it is a nonlocal event. In atomic resolution experiments, it thus opens the following question: How far is the origin of the inelastic scattering signal that is observed when the electron beam is passing nearby an atomic column or plane? We analyze computationally the delocalization of the magnetic signal in electron magnetic circular dichroism (EMCD) experiments in the so-called three-beam orientation, allowing one to image individual atomic planes. We compare the classical EMCD setup using the double-difference procedure (DD-EMCD) to a recently introduced atomic plane resolution EMCD (APR-EMCD) geometry, assuming the same probe size. We observe a strong localization of the EMCD signal to the closest atomic plane, confirming the potential of EMCD to study an evolution of magnetic properties near surfaces or interfaces with atomic plane resolution. The localization of the EMCD signal is remarkably higher than the localization of the nonmagnetic component of the inelastic scattering cross section. We also analyze double-channeling effects and find them particularly strong for the DD-EMCD method, while for APR-EMCD they appear to be minor. The DD-EMCD signal, on the other hand, appears to be more robust with respect to sample thickness than that of the APR-EMCD.

We discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. We propose that a key ingredient for an improved itinerant pairing theory is orbital selectivity, i.e., incorporating the reduced coherence of quasiparticles occupying specific orbital states. This modifies the usual spin-fluctuation theory via suppression of pair scattering processes involving those less coherent states and results in orbital selective Cooper pairing of electrons in the remaining states. We show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated iron-based superconductors.

A linear Josephson junction mediated by the surface states of a time-reversal-invariant Weyl or Dirac semimetal localizes Majorana flat bands protected by the time-reversal symmetry. We show that as a result, the Josephson current exhibits a discontinuous jump at $\pi$ phase difference which can serve as an experimental signature of the Majorana bands. The magnitude of the jump scales proportionally to the junction width and the momentum space distance between the Weyl nodes. It also exhibits a characteristic dependence on the junction orientation. We demonstrate that the jump is robust against the effects of nonzero temperature and weak nonmagnetic disorder.

We investigate the formation and dynamics of spin textures in antiferromagnetic insulators adjacent to a heavy-metal substrate with strong spin-orbit interactions. Exchange coupling to conduction electrons engenders an effective anisotropy, Dzyaloshinskii-Moriya interactions, and a magnetoelectric effect for the Néel order, which can conspire to produce nontrivial antiferromagnetic textures. Current-driven spin transfer enabled by the heavy metal, furthermore, triggers ultrafast (THz) oscillations of the Néel order for dc currents exceeding a critical threshold, opening up the possibility of terahertz spin-torque self-oscillators. For a commonly invoked antidamping-torque geometry, however, the instability current scales with the energy gap of the antiferromagnetic insulator and therefore may be challenging to reach experimentally. We propose an alternative generic geometry for inducing ultrafast autonomous antiferromagnetic dynamics.

Crystal truncation rods are used to study surface and interface structure. Since real surfaces are always somewhat miscut from a low index plane, it is important to study the effect of miscuts on crystal truncation rods. We develop a model that describes the truncation rod scattering from miscut surfaces that have steps and terraces. We show that nonuniform terrace widths and jagged step edges are both forms of roughness that decrease the intensity of the rods. Nonuniform terrace widths also result in a broad peak that overlaps the rods. We use our model to characterize the terrace width distribution and step edge jaggedness on three ${\mathrm{SrTiO}}_3$ (001) samples, showing excellent agreement between the model and the data, confirmed by atomic force micrographs of the surface morphology. We expect our description of terrace roughness will apply to many surfaces, even those without obvious terracing.