We revisit the problem of two-dimensional metals in the vicinity of a quantum phase transition to incommensurate $\mathbf{Q}=2k_F$ charge-density-wave order, where the order-parameter wave vector $\mathbf{Q}$ connects two hot spots on the Fermi surface with parallel tangents. Earlier theoretical works argued that such critical points are potentially unstable, if the Fermi surface at the hot spots is not sufficiently flat. Here we perform a controlled, perturbative renormalization-group analysis and find a stable fixed point corresponding to a continuous quantum phase transition, which exhibits a strong dynamical nesting of the Fermi surface at the hot spots. We derive scaling forms of correlation functions at the critical point and discuss potential implications for experiments with transition-metal dichalcogenides and rare-earth tellurides.

It is important to study the van der Waals interface in emerging vertical heterostructures based on layered two-dimensional (2D) materials. Being atomically thin, 2D materials are susceptible to significant strains as well as charge-transfer doping across the interfaces. Here we use Raman and photoluminescence spectroscopy to study the interface between monolayer graphene/${\mathrm{MoS}}_2$ heterostructures prepared by mechanical exfoliation and layer-by-layer transfer. By using correlation analysis between the Raman modes of graphene and ${\mathrm{MoS}}_2$, we show that both layers are subjected to compressive strain and charge-transfer doping following mechanical exfoliation and thermal annealing. Furthermore, we show that both strain and carrier concentration can be modulated in the heterostructures with additional thermal annealing. Our study highlights the importance of considering both mechanical and electronic coupling when characterizing the interface in van der Waals heterostructures, and it demonstrates a method to tune their electromechanical properties.

Combining tight-binding (TB) models with first-principles calculations, we investigate electronic and topological properties of plumbene. ‘‘Constructive’’ coupling effects of topological states are found in the low-buckled plumbene, causing the system being a normal insulator, opposite to topologically nontrivial states formed in other two-dimensional (2D) group IVA monolayers (from graphene to stanene). Local topologic states are found being located not only at the $\mathrm{\Gamma }$ point in the Brillouin zone, but also at the $K/K^{\prime }$ points, whose coupling determines the final topological behaviors of the whole system. Based on this mechanism, several schemes are raised to produce a globally topological state in the plumbene. When the plumbene is functionalized with ethynyl ($\mathrm{Pb}{\mathrm{C}}_2\mathrm{H}$), a quantum spin Hall (QSH) state comes out. Very interestingly, when an exchange field is introduced to the pristine plumbene, the system exhibits a quantum anomalous Hall (QAH) effect despite the coupling effect of the topological states.

We theoretically examine the capacitive coupling between two quantum dot hybrid qubits, each consisting of three electrons in a double quantum dot, as a function of the energy detuning of the double dot potentials. We show that a shaped detuning pulse can produce a two-qubit maximally entangling operation in $\sim 50\mathrm{ns}$ without having to simultaneously change tunnel couplings. Simulations of the entangling operation in the presence of experimentally realistic charge noise yield two-qubit fidelities over 90%.

Majorana zero modes are non-Abelian quasiparticles that emerge on the edges of topological phases of superconductors. Evidence of their presence have been reported in transport measurements on engineered superconducting-based nanostructures. In this article we identify signatures of a topologically protected dynamical manipulation of Majorana zero modes via continuous transport measurement during the manipulation. Specifically, we show that, in a two-terminal geometry, the heat pumped across the terminals at low temperature and voltage bias is characterized by a universal value. We show that this feature is inherent to the presence of Majorana zero modes and discuss its robustness against temperature, voltage bias, and the detailed coupling to the contacts.

We study the doping evolution of spin excitations in a one-dimensional (1D) Hubbard model and its downfolded spin Hamiltonians, by using exact diagonalization combined with cluster perturbation theory. In all models we observe hardening (softening) of spin excitations upon electron (hole) doping, which are reminiscent of recent experiments on two-dimensional (2D) cuprate materials. We also find that the three-site and even higher-order terms are crucial for the low-energy effective spin models to reproduce the magnetic spectra of doped Hubbard systems at a quantitative level. To interpret the numerical results, we further employ a strong coupling slave-boson mean-field theory. The mean-field theory provides an intuitive understanding of the overall compact support of dynamic spin structure factors, including the shift of zero-energy modes and change of spin excitation bandwidth with doping. Our results can serve as predictive benchmarks for future inelastic x-ray or neutron scattering experiments on doped 1D antiferromagnetic Mott insulators.

We have carried out first-principles calculations for Al(100), Al(110), and Al(111) films up to 30 monolayers to study the oscillatory quantum size effects (QSEs) exhibited in the surface energy, work function, electron-phonon coupling constants, and superconductivity transition temperature $T_c$. Significant oscillatory QSEs on these physical properties are reported. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. The oscillations in the surface energy and work function for Al(111) and Al(110) films as a function of film thickness can be well fitted by a damped sinusoidal function with the periodicity determined by one Fermi wave vector along the [111] direction or a combination of three Fermi wave vectors along the [110] direction. A quantitative description of these QSE requires the full consideration of the crystal band structure.

A transition from a cylindrical quantum dot to a highly elongated quantum dash is theoretically studied here with an atomistic approach combining empirical tight binding for single particle states and a configuration interaction method for excitonic properties. Large nanostructure shape anisotropy leads to a peculiar trend of bright exciton splitting, which is quenched at a certain point with further shape elongation, contradicting predictions of simplified models. Moreover, strong shape elongation promotes pronounced optical activity of the dark exciton, which can reach a substantial 1% of the bright exciton intensity without application of any external fields. An atomistic calculation is augmented with an elementary phenomenological model expressed in terms of light-hole exciton admixture increasing with shape deformation. Also, excitonic complexes $X^{-}, X^{+}$, and $XX$ are studied, and the correlations due to the presence of higher excited states are identified as key factors affecting excitonic binding energies and the fine structure.

Studies of possible localization of phonons in nanomaterials have gained importance in recent years in the context of thermoelectricity where phonon-localization can reduce thermal conductivity, thereby improving the efficiency of thermoelectric devices. However, despite significant efforts, phonon-localization has not yet been observed experimentally in real materials. Here we propose that surface-roughness dominated nanowires are ideal candidates to observe localization of phonons, and show numerically that the space and time evolution of the energy generated by a heat-pulse injected at a given point shows clear signatures of phonon localization. We suggest that the same configuration might allow experimental observation of localization of phonons. Our results confirm the universality in the surface-roughness dominated regime proposed earlier, which allows us to characterize the strength of disorder by a single parameter combining the width of the wire as well as the mean height of the corrugation and its correlation length.

The Rabi oscillations of a coupled spin-photon system are experimentally studied using time-domain microwave transmission measurements. An external magnetic bias field is applied to control the amplitude and frequency of Rabi oscillations, which are a measure of the coherent spin-photon energy exchange and coupling strength, respectively. The amplitude and frequency control is described in the time domain as the transient response of a two-level spin-photon system. This approach reveals the link between steady-state and transient behavior of the cavity magnon-polariton, which is necessary to interpret time-domain cavity-spintronic experiments. This transient model also allows us to understand and quantify the polariton mode composition, which can be controlled by the bias field.

We demonstrate a fast numerical method of theoretical studies of skyrmion lattice or spiral order in magnetic materials with Dzyaloshinsky-Moriya interaction. The method is based on the Fourier expansion of the magnetization combined with a minimization of the free-energy functional of the magnetic material in Fourier space, yielding the optimal configuration of the system for any given set of parameters. We employ a Lagrange multiplier technique in order to satisfy micromagnetic constraints. We apply this method to a system that exhibits, depending on the parameter choice, ferromagnetic, skyrmion lattice, or spiral (helical) order. Known critical fields corresponding to the helical-skyrmion as well as the skyrmion-ferromagnet phase transitions are reproduced with high precision. Using this numerical method we predict new types of excited (metastable) states of the skyrmion lattice, which may be stabilized by coupling the skyrmion lattice with a superconducting vortex lattice. The method can be readily adapted to other micromagnetic systems.

The surface terminations of 122-type alkaline earth metal iron pnictides $AE\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2(AE=\mathrm{Ca},\text{Ba})$ are investigated with scanning tunneling microscopy/spectroscopy. Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with an atomically resolved (√2 × √2)R45 or 1 × 2 lattice, as well as a very rare termination of 1 × 1 lattice symmetry. By analyzing the lattice registration and selective chemical marking, we identify these terminations as (√2 × √2)R45-reconstructed AE, 1 × 2-reconstructed As, and (√2 × √2)R45-reconstructed Fe surface layers, respectively. Layer-resolved tunneling spectroscopy on these terminating surfaces reveals a well-defined superconducting energy gap on the As terminations, while the gap features become weaker on the AE terminations and absent on the Fe terminations. The superconducting gap is hardly affected locally by the As or AE surface reconstructions. The definitive identification of the surface terminations and the associated spectroscopic signatures shed light on the essential roles of As and the pnictogen-iron-pnictogen trilayer building block in iron-based superconductivity.

Herein, fundamentals of topology and symmetry breaking are used to understand crystallization and geometrical frustration in topologically close-packed structures. This frames solidification from a new perspective that is unique from thermodynamic discussions. Crystallization is considered as developing from undercooled liquids, in which orientational order is characterized by a surface of a sphere in four-dimensions (quaternion) with the binary polyhedral representation of the preferred orientational order of atomic clustering inscribed on its surface. As a consequence of the dimensionality of the quaternion orientational order parameter, crystallization is seen as occurring in “restricted dimensions.” Homotopy theory is used to classify all topologically stable defects, and third homotopy group defect elements are considered to be generalized vortices that are available in superfluid ordered systems. This topological perspective approaches the liquid-to-crystalline solid transition in three-dimensions from fundamental concepts of: Bose-Einstein condensation, the Mermin-Wagner theorem and Berezinskii-Kosterlitz-Thouless (BKT) topological-ordering transitions. In doing so, in this article, concepts that apply to superfluidity in “restricted dimensions” are generalized to consider the solidification of crystalline solid states.

Here we present the structural and magnetic properties of the new honeycomb material ${\mathrm{Ag}}_3{\mathrm{LiMn}}_2{\mathrm{O}}_6$. The system $\mathrm{Ag}\left[{\mathrm{Li}}_{1/3}{\mathrm{Mn}}_{2/3}\right]{\mathrm{O}}_2$ belongs to a quaternary $3R$-delafossite family and crystallizes in a monoclinic symmetry with space group $C2/m$, and the magnetic ${\mathrm{Mn}}^{4+}(S=3/2)$ ions form a honeycomb network in the $ab$ plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature ${\theta }_{CW}\sim -51\mathrm{K}$) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48 K, and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. An asymmetric Bragg peak (characteristic of two-dimensional order), seen in neutron diffraction, gains intensity even above the ordering temperature, thus showing the existence of short-range spin correlations. Our electron spin resonance (ESR) experiments corroborate the bulk magnetic data. Additionally, the ESR line broadening upon approaching the ordering temperature $T_{\mathrm{N}}$ could be described in terms of a Berezinskii-Kosterlitz-Thouless scenario with $T_{\mathrm{KT}}=40\left(1\right)\mathrm{K}$. The ${}^7\mathrm{Li}$ NMR line shift probed as a function of temperature tracks the static susceptibility ($K_{\mathrm{iso}}$) of magnetically coupled ${\mathrm{Mn}}^{4+}$ ions. The ${}^7\mathrm{Li}$ spin-lattice relaxation rate ($1/T_1$) exhibits a sharp decrease below about 50 K. A critical divergence is absent at the ordering temperature perhaps because of the filtering out of the antiferromagnetic fluctuations at the Li site, i.e., at the centers of the hexagons in the honeycomb network. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system ${\mathrm{Ag}}_3{\mathrm{LiMn}}_2{\mathrm{O}}_6$. Our ab initio electronic structure calculations suggest that in the $ab$ plane, the nearest-neighbor (NN) exchange interaction is strong and AFM, while the next-NN and the third-NN exchange interactions are FM and AFM, respectively. The interplanar exchange interaction is found to be relatively small. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.

First-principles calculations and effective model analysis are used to prove the existence of a single pair of three-dimensional Dirac points in the well-known layered titanium-based oxypnictide superconductor ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$. The nontrivial topological character of ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$ is demonstrated by calculating the surface states. The Dirac points, residing on the rotational axis, are found approximately 70 meV below the Fermi level which can be lifted to Fermi surface by Sb doping. The Dirac points and related physics are expected to be detected by angle-resolved photoemission spectroscopy. This compound is also found to be a superconductor with transition temperature $\left(T_c\right)4$.6 K; our results may provide an appropriate platform to study the interplay between topological Dirac fermions and superconductivity.

Recent numerical results [Gonzalez et al., Phys. Rev. Lett. 122, 017201 (2019); Shimada et al., J. Phys. Conf. Ser. 969, 012126 (2018)] point to the existence of a partial-disorder ground state for a spin-1/2 antiferromagnet on the stuffed honeycomb lattice, with 2/3 of the local moments ordering in an antiferromagnetic Néel pattern, while the remaining 1/3 of the sites display short-range correlations only, akin to a quantum spin liquid. We derive an effective model for this disordered subsystem, by integrating out fluctuations of the ordered local moments, which yield couplings in a formal $1/S$ expansion, with $S$ being the spin amplitude. The result is an effective triangular-lattice XXZ model, with planar ferromagnetic order for large $S$ and a stripe-ordered Ising ground state for small $S$, the latter being the result of frustrated Ising interactions. Within the semiclassical analysis, the transition point between the two orders is located at $S_c=0.646$, being very close to the relevant case $S=1/2$. Near $S=S_c$ quantum fluctuations tend to destabilize magnetic order. We conjecture that this applies to $S=1/2$, thus explaining the observed partial-disorder state.

Employing a combination of a sign-problem-free Monte Carlo approach and nonequilibrium Green's-function techniques, we study nonequilibrium charge transport in a model heterostructure, where a two-dimensional spinless Falicov-Kimball system is coupled to two noninteracting leads. We show that the transport characteristic depends sensitively on the electrostatic potential in the system and exhibits different properties for different phases of the Falicov-Kimball model. In particular, pronounced steplike changes of the current and transmission are observed at the phase boundaries, evident even on a logarithmic scale. Analyzing finite-size effects, we find that with the method used a relatively small system can be utilized to address specific thermodynamic limits. Phenomenon of the localization is discussed as well.

A key notion in heavy-fermion systems is the entanglement between conduction electrons and localized spin degrees of freedom. To study these systems from this point of view, we compute the mutual information in a ferromagnetic and antiferromagnetic Kondo lattice model in the presence of geometrical frustration. Here the interplay between the Kondo effect, the Ruderman-Kittel-Kasuya-Yosida interaction, and geometrical frustration leads to partial Kondo screened, conventional Kondo insulating, and antiferromagnetic phases. In each of these states the mutual information follows an area law, the coefficient of which shows sharp crossovers (on our finite lattices) across phase transitions. Deep in the respective phases, the area law coefficient can be understood in terms of simple direct product wave functions thereby yielding an accurate measure of the entanglement in each phase. The above-mentioned results stem from approximation-free auxiliary field quantum Monte Carlo simulations.

A hybrid system composed by a semiconducting nanowire with proximity-induced superconductivity and a quantum dot at the end working as a spectrometer was recently used to quantify the so-called degree of Majorana nonlocality [M.-T. Deng et al. Phys. Rev. B 98, 085125 (2018)]. Here we demonstrate that the spin-resolved density of states of the dot responsible for the zero-bias conductance peak strongly depends on the separation between the Majorana bound states and their relative couplings with the dot and investigate how the charging energy affects the spectrum of the system in the distinct scenarios of Majorana nonlocality (topological quality). Our findings suggest that the spin-resolved spectroscopy of the local density of states of the dot can be used as a powerful tool for discriminating between different scenarios of the emergence of the zero-bias conductance peak.