We combined optical spectroscopy with first principles calculations to reveal the electronic signatures of Ir dimer formation in the $q=(1/5,0,1/5)$ high resistivity phase of ${\mathrm{IrTe}}_2$. Our measurements uncover two interband transitions into the unoccupied $d_{xy}$ antibonding orbital, one from mixed iridium/tellurium bands, the other from the $d_{xy}$ bonding orbital of the dimerized Ir centers. The bonding-antibonding splitting demonstrates that iridium, not tellurium, plays the dominant role in stabilizing the low temperature phase of ${\mathrm{IrTe}}_2$ through localized bonding orbital formation.

We theoretically investigate the time dependence of the first-order coherence function for a one-dimensional driven dissipative nonequilibrium condensate. Simulations on the generalized Gross-Pitaevskii equation show that the characteristic time scale of exponential decay agrees with the linearized Bogoliubov theory in the regime of large interaction energy. For very weak interactions, the temporal correlation deviates from the linear theory, and instead respects the dynamic scaling of the Kardar-Parisi-Zhang universality class. This nonlinear dynamics is found to be quantitatively captured by a noisy Kuramoto-Sivashinsky equation for the phase dynamics.

We investigate the topological and transport properties of the recently discovered valley-polarized quantum anomalous Hall (VQAH) phase. In a single layer, the phase is realized through the competition between two types of spin-orbit coupling, which breaks the symmetry between the two valleys. We show that the topological phase transition from conventional quantum anomalous Hall phase to the VQAH phase is due to the change of topological charges with the generation of additional skyrmions in the real spin texture, when the band gap closes and reopens at one of the valleys. In the presence of short-range disorders, pairs of the gapless edge channels (one from each valley in a pair) would be destroyed due to intervalley scattering. However, we discover that in an extended range of moderate scattering strength, the transport through the system is quantized and fully valley polarized, i.e., the system is equivalent to a quantum anomalous Hall system with valley-filtered chiral edge channels. We further show that with an additional layer degree of freedom, a much richer phase diagram could be realized with multiple VQAH phases. For a bilayer system, we demonstrate that topological phase transitions could be controlled by the interlayer bias potential.

We study resonant tunneling through a periodic square array of quantum dots sandwiched between modulation-doped quantum wells. If a magnetic field is applied parallel to the quantum dot plane, the tunneling current exhibits a highly complex Aharonov-Bohm oscillation pattern due to the interference of multiple pathways traversed by a tunneling electron. Individual pathways associated with conductance beats can be enumerated by sweeping the magnetic field at various tilt angles. Remarkably, Aharonov-Bohm oscillations are aperiodic unless the magnetic field slope relative to the quantum dot lattice axes is a rational number.

This paper provides a method based on rigorous coupled wave analysis for the calculation of the radiative thermal conductance between a layer that is patterned with arbitrary, periodically repeating features and a planar substrate. This method is applied to study the transfer from an array of beams with a rectangular cross section. The impact of the structure size and spacing on the thermal conductance are investigated. These calculations are compared to an effective medium theory, which becomes increasingly accurate as the structure sizes fall well below the relevant resonance wavelengths of materials and structures. Moreover, comparisons are made with a modified proximity approximation and the far-field approximation, which become valid for small and large spacings, respectively. Results show that new levels of control over the magnitude and spectral contributions to thermal conductance can be achieved with corrugated structures relative to planar ones. Specifically, we show for SiC arrays with rectangular cross sections and with the same filling fraction, that the use of a smaller periodicity leads to a lowered far-field thermal transfer and an increased near-field thermal transfer.

We study square lattice space-group symmetry fractionalization in a family of exactly solvable models with ${\mathbb{Z}}_2$ topological order in two dimensions. In particular, we have obtained a complete understanding of which distinct types of symmetry fractionalization (symmetry classes) can be realized within this class of models, which are generalizations of Kitaev's ${\mathbb{Z}}_2$ toric code to arbitrary lattices. This question is motivated by earlier work of Essin and one of us (M. H.) [Phys. Rev. B 87, 104406 (2013)], where the idea of symmetry classification was laid out, and which, for square lattice symmetry, produces 2080 symmetry classes consistent with the fusion rules of ${\mathbb{Z}}_2$ topological order. This approach does not produce a physical model for each symmetry class, and indeed there are reasons to believe that some symmetry classes may not be realizable in strictly two-dimensional systems, thus raising the question of which classes are in fact possible. While our understanding is limited to a restricted class of models, it is complete in the sense that for each of the 2080 possible symmetry classes, we either prove rigorously that the class cannot be realized in our family of models, or we give an explicit model realizing the class. We thus find that exactly 487 symmetry classes are realized in the family of models considered. With a more restrictive type of symmetry action, where space-group operations act trivially in the internal Hilbert space of each spin degree of freedom, we find that exactly 82 symmetry classes are realized. In addition, we present a single model that realizes all $2^6=64$ types of symmetry fractionalization allowed for a single anyon species (${\mathbb{Z}}_2$ charge excitation), as the parameters in the Hamiltonian are varied. The paper concludes with a summary and a discussion of two results pertaining to more general bosonic models.

The critical behavior of the single-crystal helimagnet MnSi is investigated by means of bulk dc magnetization at the boundary between the conical state and paramagnetic phase. We obtain the critical exponents $(\beta =0.242\pm 0.006,\gamma =0.915\pm 0.003$, and $\delta =4.734\pm 0.006)$, where the self-consistency and reliability are verified by the Widom scaling law and the scaling equation. The critical exponents of MnSi belong to the universality class of tricritical mean-field theory, which unambiguously indicates a tricritical phenomenon at the boundary between the first-order phase transition and the second-order one induced by the external magnetic field. The tricritical point (TCP) is determined as $H_{\mathrm{TCP}}\approx 3200$ Oe at the critical temperature, consistent with the previous report [A. Bauer, M. Garst, and C. Pfleiderer, Phys. Rev. Lett. 110, 177207 (2013)]. The critical behavior suggests a long-range magnetic coupling with the exchange distance decaying as $J\left(r\right)\approx r^{-4.3}$ in MnSi.

We report measurements and theoretical analysis of resonant spin tunneling in randomly oriented nanospheres of a molecular magnet. Amorphous nanospheres of ${\mathrm{Mn}}_{12}$ acetate have been fabricated and characterized by chemical, infrared, TEM, x-ray, and magnetic methods. Magnetic measurements have revealed sharp tunneling peaks in the field derivative of the magnetization that occur at the typical resonant field values for the ${\mathrm{Mn}}_{12}$ acetate crystal in the field parallel to the easy axis. Theoretical analysis is provided that explains these observations. We argue that resonant spin tunneling in a molecular magnet can be established in a powder sample, without the need for a single crystal and without aligning the easy magnetization axes of the molecules. This is confirmed by reanalyzing the old data on a powdered sample of nonoriented micron-size crystals of ${\mathrm{Mn}}_{12}$ acetate. Our findings can greatly simplify the selection of candidates for quantum spin tunneling among newly synthesized molecular magnets.

The in-plane thermal conductivity of iron-based superconductor ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ single crystal $(T_c\approx$ 2.1 K) was measured down to 100 mK. In zero field, the observation of a significant residual linear term ${\kappa }_0/T$ = 0.65 mW ${\mathrm{K}}^{-2}$ ${\mathrm{cm}}^{-1}$ provides clear evidence for nodal superdonducting gap. The field dependence of ${\kappa }_0/T$ is similar to that of its sister compound ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ with comparable residual resistivity ${\rho }_0$ and lies between the dirty and clean ${\mathrm{KFe}}_2{\mathrm{As}}_2$. These results suggest that the (K, Rb, Cs)${\mathrm{Fe}}_2{\mathrm{As}}_2$ serial superconductors have a common nodal gap structure.

In addition to charge plasmons, a 2D electron system with Rashba-type spin-orbit coupling (SOC) also supports three collective modes in the spin sector: the chiral-spin modes. We study the dispersions of the charge and spin modes and their coupling to each other within a generalized random phase approximation for arbitrarily strong SOC, and both in 2D and 3D systems. In both 2D and 3D, we find that the charge plasmons are coupled to only one of the three chiral-spin modes. This coupling is shown to affect the dispersions of the modes at finite but not at zero wave numbers. In 3D, the chiral-spin modes are strongly damped by particle-hole excitations and disappear for weak electron-electron interaction. The Landau damping of the chiral-spin modes in 3D is directly related to the fact that, in contrast to 2D, there is no gap for particle-hole excitations between spin-split subbands. The gapless continuum is also responsible for the Landau damping of the charge plasmon in 3D—a qualitatively new feature of the SOC system. We also discuss the optical conductivity of clean 2D and 3D systems and show that SOC introduces spectral weight at finite frequency in a such way that the sum rule is satisfied. The in-plane transverse chiral-spin mode shows up as a dispersing peak in the optical conductivity at finite number, which can be measured in the presence of diffraction grating. We also discuss possible experimental manifestations of chiral-spin modes in semiconductor quantum wells such InGaAs/AlGaAs and 3D giant Rashba materials of the BiTeI family.

A recently published generalized gradient approximation functional within density functional theory (DFT) has shown, in a few paradigm tests, an improved KS orbital description over standard (semi)local approximations. The characteristic feature of this functional is an enhancement factor that diverges like $sln\left(s\right)$ for large reduced density gradients $s$ which leads to unusual properties. We explore the improved orbital description of this functional more thoroughly by computing the electronic band structure, band gaps, and the optical dielectric constants in semiconductors, Mott insulators, and ionic crystals. Compared to standard semilocal functionals, we observe improvement in both the band gaps and the optical dielectric constants. In particular, the results are similar to those obtained with orbital functionals or by perturbation theory methods in that it opens band gaps in systems described as metallic by standard (semi)local density functionals, e.g., Ge, $\alpha$-Sn, and CdO.

The recent experimental advances in capacitively coupled singlet-triplet qubits, particularly the demonstration of entanglement, opens the question of what type of entangling gates the system's Hamiltonian can produce directly via a single square pulse. We address this question by considering the system's Hamiltonian from first principles and using the representation of its nonlocal properties in terms of local invariants. In the analysis we include the three different ways in which the system can be biased and their effect on the generation of entangling gates. We find that, in one of the possible biasing modes, the Hamiltonian has an especially simple form, which can directly generate a wide range of different entangling gates including the iswap gate. Moreover, using the complete form of the Hamiltonian we find that, for any biasing mode, a cnot gate can be generated directly.

We propose a practical scheme to generate a pure valley current in monolayer transition-metal dichalcogenides by one-photon absorption of linearly polarized light. We show that the pure valley current can be detected by either photoluminescence measurements or the ultrafast pump-probe technique. Our method, together with the previously demonstrated generation of valley polarization, opens up the exciting possibility of ultrafast optical-only manipulation of the valley index. The tilted field effect on the valley current in experiment is also discussed.

Dispersionless (flat) electronic bands are investigated regarding their conductance properties. Due to “caging” of carriers these bands are usually insulating at partial filling, at least on a noninteracting level. Considering the specific example of a ${\mathcal{T}}_3$ lattice, we study long-range Coulomb interactions. A nontrivial dependence of the conductivity on flat-band filling is obtained, exhibiting an infinite number of zeros. Near these zeros, the conductivity rises linearly with carrier density. At densities halfway in between adjacent conductivity zeros, strongly enhanced conductivity is predicted, accompanying a solid-solid phase transition.

We identify the existence of various symmetry-protected topological states in one-dimensional superlattices with periodically modulated hopping amplitudes or on-site potentials, which can be characterized by the quantized Berry phase $\pi$ or the emergence of a pair of degenerate boundary states. It is shown that there may exist three types of topological phases, which are protected by inversion symmetry, chiral symmetry, and both of them, respectively, depending on the modulations, the odd, or the even modulation period. The connection between the hopping and potential modulations is also discussed. Furthermore, we demonstrate that the topological phase protected by the inversion symmetry can be realized in the interacting boson systems trapped in the same superlattices. The results can very possibly be studied experimentally in superlattice systems engineered with state-of-the-art technologies.

The electronic structure of scandium nitride is determined by combining results from optical and electronic transport measurements with first-principles calculations. Hybrid functional Heyd-Scuseria-Ernzerhof (HSE06) calculations indicate a 0.92 eV indirect Γ to $X$ band gap and direct transition energies of 2.02 and 3.75 eV at $X$ and Γ points, respectively, while $G_{\mathrm{o}}{W}_{\mathrm{o}}$ and $G{W}_{\mathrm{o}}$ methods suggest 0.44–0.74 eV higher gap values. Epitaxial ScN(001) layers deposited on MgO(001) substrates by reactive sputtering exhibit degenerate $n\text{-type}$ semiconductor properties with a temperature-independent electron density that is varied from $N=1.12-12.8\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ using fluorine impurity doping. The direct optical gap increases linearly with $N$ from 2.18 to 2.70 eV, due to a Burstein-Moss effect. This strong dependence on $N$ is likely the cause for the large range (2.03–3.2 eV) of previously reported gap values. However, here extrapolation to $N=0$ yields $2.07\pm 0.05\mathrm{eV}$ for the direct $X$ point transition of intrinsic ScN. A reflection peak at $3.80\pm 0.02\mathrm{eV}$ is independent of $N$ and in perfect agreement with the HSE06-predicted peak at 3.79 eV, associated with a high joint density of states (DOS) near the Γ point. The electron mobility at 4 K is $100\pm 30\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ and decreases with temperature due to scattering at polar optical phonons with characteristic frequencies that decrease from 620 to $440\pm 30{\mathrm{cm}}^{-1}$, with increasing $N$, due to free carrier screening. The transport and DOS electron effective mass, determined from measured intra- and interband transitions, respectively, are $0.40\pm 0.02m_{\mathrm{o}}$ and $0.33\pm 0.02m_{\mathrm{o}}$, in good agreement with the first-principles predictions of $m_{tr}=0.33\pm 0.05m_{\mathrm{o}}$ and $m_{\mathrm{DOS}}=0.43\pm 0.05m_{\mathrm{o}}$. The ScN refractive index increases with increasing $h\nu =1.0-2.0\mathrm{eV}$ from 2.6–3.1 based on optical measurements and from 2.8–3.2 based on the calculated dielectric function. An overall comparison of experiment and simulation indicates (i) an overestimation of band gaps by $\mathit{GW}$ methods, but (ii) excellent agreement with a deviation of ≤0.05 eV for the hybrid functional and (iii) a value for the fundamental indirect gap of ScN of $0.92\pm 0.05\mathrm{eV}$.