From a nanoscale heat transfer point of view, currently one of the most interesting and challenging tasks is to quantitatively analyze phonon mode specific transport properties in solid materials, which plays a vital role in many emerging and diverse technological applications. It has not been long that such information can be provided by the phonon spectral energy density (SED) or equivalently time domain normal mode analysis (TDNMA) methods in the framework of equilibrium molecular dynamics (EMD) simulations. However, few methods have been developed for nonequilibrium molecular dynamics (NEMD) simulations [Phys. Rev. B 91, 115426 (2015)], the other widely used computational method for calculating thermal transport of materials in addition to EMD. In this work, a computational scheme based on time Fourier transform of atomistic heat current, called the frequency domain direct decomposed method (FDDDM), is proposed to analyze the contributions of frequency dependent thermal conductivity in NEMD simulations. The FDDDM results of Lennard-Jones argon and Stillinger-Weber Si are compared with the TDNMA method from EMD simulation. Similar trends are found for both cases, which confirm the validity of our FDDDM approach. Benefiting from the inherent nature of NEMD and the theoretical formula that does not require any temperature assumption, the FDDDM can be directly used to investigate the size and temperature effect. Moreover, the unique advantage of FDDDM prior to previous methods (such as SED and TDNMA) is that it can be straightforwardly used to characterize the phonon frequency dependent thermal conductivity of disordered systems, such as amorphous materials. The FDDDM approach can also be a good candidate to be used to understand the phonon behaviors and thus provides useful guidance for designing efficient structures for advanced thermal management.

Quantum Hall bilayer phase diagram with respect to interlayer distance bears a remarkable similarity with phase diagrams of strongly correlated systems as a function of doping, with magnetic ordering on the one end and Fermi-liquid-like behavior on the other. Moreover, it has been suggested [Phys. Rev. Lett. 101, 176803 (2008)] that a BCS correlated state of composite fermions with $p$-wave pairing may exist in the intermediate region. In the same region, an exact diagonalization study in the torus geometry [Phys. Rev. B 69, 045319 (2004)] pointed out the existence of state(s) with pseudospin spiraling order. Here we reconcile these two descriptions of the intermediate state by considering the underlying bosonic representation of the composite fermion paired state in the long-distance limit, and by performing extensive exact diagonalizations on the torus. We argue that the spiraling states belong to the manifold of degenerate ground state(s), and are a consequence of Bose condensation of the quasiparticles (with critical algebraic correlations) at nonzero momenta in the two pseudospin states. The spiraling states, generated in this way as spin textures, can be identified with meron-antimeron constructions. Thus, merons—the fractionally charged vortex excitations of the $\mathit{XY}$ magnetically ordered state—constitute some of the topological sectors. It follows that merons are deconfined in the intermediate state, and allow for a smooth transition between the magnetically ordered and Fermi-liquid-like phases, in which they are bound in pairs.

In thermodynamics a macroscopic state of a system results from a number of its microscopic states. This number is given by the exponent of the system's entropy $exp\left(S\right)$. In noninteracting systems with discrete energy spectra, such as large scale quantum dots, $S$ as a function of the temperature has usually a plateau shape with integer values of $exp\left(S\right)$ on these plateaus. Plateaus with noninteger values of $exp\left(S\right)$ are fundamentally forbidden and would be thermodynamically infeasible. Here we investigate the entropy of a noninteracting quantum dot coupled via tunneling to normal metals with continuum spectra as well as to topological superconductors. We show that the entropy may have noninteger plateaus if the topological superconductors support weakly overlapping Majorana bound states. This brings a fundamental change in the thermodynamics of the quantum dot whose specific heat $c_V$ acquires low-temperature Majorana peaks which should be absent according to the conventional thermodynamics. We also provide a fundamental thermodynamic understanding of the transport properties, such as the linear conductance. In general our results show that the thermodynamics of systems coupled to Majorana modes represents a fundamental physical interest with diverse applications depending on versatility of possible coupling mechanisms.

One of the most distinctive features of graphene is its huge inter-Landau-level splitting in experimentally attainable magnetic fields which results in the room-temperature quantum Hall effect. In this paper we calculate the longitudinal conductivity induced by two-phonon scattering in graphene in a quantizing magnetic field at elevated temperatures. It is concluded that the purely phonon-induced scattering, negligible for conventional semiconductor heterostructures under quantum Hall conditions, becomes comparable to the disorder-induced contribution to the dissipative conductivity of graphene in the quantum Hall regime.

We theoretically analyze a system where two electrons are trapped separately in two quantum dots on a suspended carbon nanotube (CNT), subject to external ac electric driving. An indirect mechanically induced coupling of two distant single electron spins is induced by the interaction between the spins and the mechanical motion of the CNT. We show that a two-qubit iswap gate and arbitrary single-qubit gates can be obtained from the intrinsic spin-orbit coupling. Combining the iswap gate and single-qubit gates, maximally entangled states of two spins can be generated in a single step by varying the frequency and the strength of the external electric driving field. The spin-phonon coupling can be turned off by electrostatically shifting the electron wave function on the nanotube.

Topological insulators doped with transition metals have recently been found to host a strong ferromagnetic state with perpendicular to plane anisotropy as well as support a quantum Hall state with edge channel transport, even in the absence of an external magnetic field. It remains unclear, however, why a robust magnetic state should emerge in materials of relatively low crystalline quality and dilute magnetic doping. Indeed, recent experiments suggest that the ferromagnetism exhibits at least some superparamagnetic character. We report on transport measurements in a sample that shows perfect quantum anomalous Hall quantization, while at the same time exhibits traits in its transport data which suggest inhomogeneities. We speculate that this may be evidence that the percolation path interpretation used to explain the transport during the magnetic reversal may actually have relevance over the entire field range.

Because of their weak coupling to the electromagnetic field, dark excitons in semiconductor quantum dots possess extremely long lifetimes, which makes them attractive candidates for quantum information processing. On the other hand, the preparation and manipulation of dark states is challenging, because commonly used optical excitation mechanisms are not applicable. We propose an efficient mechanism for the deterministic preparation of the dark exciton exploiting the application of a tilted magnetic field and the optical excitation with a chirped, i.e., frequency modulated, laser pulse.

We have mapped the potential energy landscape for a Ag atom around Ag clusters on Si(111), via positioning a probing atom in the vicinity of clusters and monitoring the subsequent thermal motions with scanning tunneling microscopy. Both the probing atom's modulated diffusion and its combination with the clusters are quantitatively measured in a wide temperature range, showing strong dependence on the cluster size as well as specific sites. The atom-cluster interactions, as large as over 200 meV, are determined and found to alternate between attraction and repulsion at different separations. The demonstrated ability of studying atomic scale dynamics of single adsorbates in complex environments can provide opportunities to understand various phenomena in nanostructure growth and nanocatalysis.

By means of first-principles calculations in combination with universal evolutionary structure search, we identified the crystalline structure of long-term argued $M_3{\mathrm{Bi}}_2$ ($M=\text{Sr}$, Ca, and Ba), which crystallizes in a tubelike structure stacked by buckled graphenelike layers. The analyses of electronic structures revealed that this type of $M_3{\mathrm{Bi}}_2$ is a native wide-gap three-dimensional topological insulator with the inverted band order induced mainly by crystal field effect. The spin-orbit coupling effect was found to open the band gap and further enhance the band inversion. Among them, ${\mathrm{Sr}}_3{\mathrm{Bi}}_2$ is most attractive due to its largest fundamental gap of about 0.3 eV and the directly inverted band gap of 0.81 eV at $\mathrm{\Gamma }$ obtained within the framework of Green functionals $\left(GW\right)$. Moreover, the computation also evidences that their tubelike structure is suitable for further treatment via magnetic dopants, which prefer to occupy $1b$ site. Interestingly, the ferromagnetic insulating state has been achieved for V- and Mn-doped cases. This may provide a further opportunity to observe the quantized anomalous Hall effect in its thin films.

We present an effective ${\mathbb{Z}}_2$ gauge theory that captures various competing phases in spin-1/2 kagome lattice antiferromagnets: the topological ${\mathbb{Z}}_2$ spin liquid (SL) phase, and the 12-site and 36-site valence bond solid (VBS) phases. Our effective theory is a generalization of the recent ${\mathbb{Z}}_2$ gauge theory proposed for SL phases by Wan and Tchernyshyov. In particular, we investigate possible VBS phases that arise from vison condensations in the SL. In addition to the 12-site and 36-site VBS phases, there exists 6-site VBS that is closely related to the symmetry-breaking valence bond modulation patterns observed in the recent density matrix renormalization group simulations. We find that our results have remarkable consistency with a previous study using a different ${\mathbb{Z}}_2$ gauge theory. Motivated by the lattice geometry in the recently reported vanadium oxyfluoride kagome antiferromagnet, our gauge theory is extended to incorporate lowered symmetry by inequivalent up and down triangles. We investigate effects of this anisotropy on the 12-site, 36-site, and 6-site VBS phases. The 12-site VBS is stable to anisotropy while the 36-site VBS undergoes severe dimer melting. Interestingly, any analog of the 6-site VBS is not found in this approach. We discuss the implications of these findings and also compare the results with a different type of ${\mathbb{Z}}_2$ gauge theory used in previous studies.

While the stoichiometric intermetallic compound FeAl is found to be paramagnetic in experiment, standard band-theory approaches predict the material to be ferromagnetic. We show that this discrepancy can be overcome by a better treatment of electronic correlations with density-functional plus dynamical mean-field theory. Our results show no ferromagnetism down to $100\mathrm{K}$ and since the susceptibility is decreasing at the lowest temperatures studied we also do not expect ferromagnetism at even lower temperatures. This behavior is found to originate from temporal quantum fluctuations that screen short-lived local magnetic moments of $1.6{\mu }_{\mathrm{B}}$ on Fe.

We report a theoretical prediction of a new class of bulk and intrinsic quantum anomalous Hall (QAH) insulators $\mathrm{La}X$ $(X$=Br, Cl, and I) via relativistic first-principles calculations. We find that these systems are innate long-ranged ferromagnets which, with the help of intrinsic spin-orbit coupling, become QAH insulators. A low-energy multiband tight-binding model is developed to understand the origin of the QAH effect. Finally, integer Chern number is obtained via Berry phase computation for each two-dimensional plane. These materials have the added benefit of a sizable band gap of as large as $\sim 25$ meV, with the flexibility of enhancing it to above 75 meV via strain engineering. The synthesis of $\mathrm{La}X$ materials will provide the impurity-free single crystals and thin-film QAH insulators for versatile experiments and functionalities.

Via angular Shubnikov–de Haas (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs, a Weyl semimetal candidate. The SdH oscillations consist of two frequencies corresponding to two Fermi surface extrema: 20.8 T $(\alpha$ pocket) and 15.6 T $(\beta$ pocket). The analysis, including a Landau fan plot, shows that the $\beta$ pocket has a Berry phase of $\pi$ and a small effective mass of $\sim 0.033m_0$, indicative of a nontrivial topology in momentum space, whereas the $\alpha$ pocket has a trivial Berry phase of 0 and a heavier effective mass of $\sim 0.066m_0$. From the effective mass and the $\beta$-pocket frequency, we determine that the Weyl node is 110.5 meV from the chemical potential. An electron-hole compensation effect is discussed in this system, and its impact on magnetotransport properties is addressed. The difference between NbAs and other monopnictide Weyl semimetals is also discussed.

The spin relaxation due to the spin-orbit interaction (SOI) is studied theoretically in a quantum well with electrons occupying only the ground subband. First, it is shown that the coefficient of the Rashba SOI is proportional to $b_{\mathrm{off}}-1$, in which the parameter $b_{\mathrm{off}}$, determined by the band offsets and the band gaps, passes through unity, for example, by changing $x$ in ${\mathrm{Ga}}_{0.47}{\mathrm{In}}_{0.53}\mathrm{As}$(well)/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{\mathrm{As}}_y{\mathrm{Sb}}_{1-y}$(barrier). Second, it is derived that the transition matrix element of each spin-flip phonon scattering has the same proportionality factor $b_{\mathrm{off}}-1$, in addition to the impurity scattering previously studied by the same authors [Phys. Rev. B 89, 075314 (2014)]. These findings suggest the possibility of strongly suppressing the spin-relaxation rate by choosing appropriate materials.

We report an experimental verification of the recent prediction that sharp transitions between steady states in multistable cavity-polariton systems are mediated by intermode parametric scattering that triggers the accumulation of energy and, hence, lowers the threshold at the cost of extending the transition latency period [S. S. Gavrilov, Phys. Rev. B 90, 205303 (2014)]. The time-resolved measurements are performed using a high-$Q$ GaAs microcavity pumped slightly above the lower polariton level at normal incidence.

We have measured magnetic field dependences of the attenuation and velocity of surface acoustic waves in a high-mobility $n$-GaAs/AlGaAs structure with a wide quantum well. The results allowed us to find the complex conductance $\sigma \left(\omega \right)$ of the heterostructure for different frequencies, temperatures, and magnetic fields near filling factors $\nu =1,2$. Observed behavior of $\sigma \left(\omega \right)$ versus magnetic field outside close vicinities of integer fillings reveals an oscillation pattern indicative of the rich fractional quantum Hall effect. Our result is that in very close vicinities of integer filling factors the AC response of a high-mobility two-dimensional structures behaves as that of a two-dimensional system of localized electrons. Namely, both real and imaginary parts of the complex AC conductance at low temperatures agree with the predictions for the two-site model for a two-dimensional hopping system. Another result is the specific temperature dependences of $\sigma \left(\omega \right)$, which are extremely sensitive to the filling factor value. These dependences indicate a sharp crossover between the localized modes and a pinned Wigner crystal.

We present a theoretical analysis showing that layered topological insulators, for example, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ are optically hyperbolic materials in the range of terahertz (THz) frequencies. As such, these topological insulators possess deeply subdiffractional, highly directional collective modes: hyperbolic phonon polaritons. We predict that in thin crystals the dispersion of these modes is split into discrete subbands and is strongly influenced by electron surface states. If the surface states are doped, then hybrid collective modes result from coupling of the phonon polaritons with surface plasmons. The strength of the hybridization can be controlled by an external gate that varies the chemical potential of the surface states. We also show that the momentum dependence of the plasmon-phonon coupling leads to a polaritonic analog of the Goos-Hänchen effect. The directionality of the polaritonic rays and their tunable Goos-Hänchen shift is observable via THz nanoimaging.

I identify a nontrivial four-state Landau-Zener model for which transition probabilities between any pair of diabatic states can be determined analytically and exactly. The model describes an experimentally accessible system of two interacting qubits, such as a localized state in a Dirac material with both valley and spin degrees of freedom or a singly charged quantum dot (QD) molecule with spin orbit coupling. Application of the linearly time-dependent magnetic field induces a sequence of quantum level crossings with possibility of interference of different trajectories in a semiclassical picture. I argue that this system satisfies the criteria of integrability in the multistate Landau-Zener theory, which allows one to derive explicit exact analytical expressions for the transition probability matrix. I also argue that this model is likely a special case of a larger class of solvable systems, and present a six-state generalization as an example.

We analyze spin scattering in ballistic transport of electrons through a ripple at a normal incidence of an electron flow. The model of a ripple consists of a curved graphene surface in the form of an arc of a circle connected from the left-hand and right-hand sides to two flat graphene sheets. At certain conditions the curvature-induced spin-orbit coupling creates a transparent window for incoming electrons with one spin polarization simultaneously with a backscattering of those with opposite polarization. This window is equally likely transparent for electrons with spin up and spin down that move in opposite directions. The spin-filtering effect that is small in one ripple becomes prominent with the increase of $N$ consequently connected ripples that create a graphene sheet of the sinusoidal type. We present the analytical expressions for spin-up and spin-down transmission probabilities as a function of $N$ connected ripples.

We show that the $d^0$ ferromagnetism with high Curie temperature $\left(T_c\right)$ can be achieved in the electron-doped hydrogenated epitaxial graphene on certain SiC substrates through first-principles calculations. The pristine systems are found to be Mott insulators regardless of SiC polytype $(2H,4H$, or $6H)$ which, however, plays a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double-exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A $T_c$ of around 400 K is predicted for graphene on the $2H$-SiC substrate. We employ a nondegenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.