${\mathrm{PtBi}}_2$ with a layered hexagonal crystal structure was recently reported to exhibit an unconventional large linear magnetoresistance, while the mechanism involved is still elusive. Using high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on its bulk and surface electronic structure. Through careful comparison with first-principle calculations, our experiment distinguishes the low-lying bulk bands from entangled surface states, allowing the estimation of the real composition of samples. We find significant electron doping in ${\mathrm{PtBi}}_2$, implying a substantial Bi-deficiency-induced disorder therein. Intriguingly, we discover a Dirac-cone-like surface state on the boundary of the Brillouin zone, which is identified as an accidental Dirac band without topological protection. Our findings exclude linear band dispersion in the quantum limit as the cause of the unconventional large linear magnetoresistance but give support to the classical disorder model from the perspective of the electronic structure.

By employing the $G_0{W}_0$ approximation of Hedin's $GW$ approach one can obtain quasiparticle energies of extended systems and molecules with good accuracy. However, for many materials, semilocal exchange-correlation functionals are unsatisfactory starting points for $G_0{W}_0$ calculations. Hybrid functionals often improve upon them, but at a substantially higher computational cost. As an alternative, we suggest the LDA-1/2 method, which provides reasonable band gaps without being computationally involved. In this work, we systematically compare three starting points for $G_0{W}_0$: LDA, PBE0, and LDA-1/2. A selection of solids is chosen for this benchmark: C, Si, SiC, AlP, LiF, MgO, Ne, Ar, GaN, GaAs, CdS, ZnS, and ZnO. We demonstrate that LDA-1/2 is a good starting point in most cases, reducing the mean absolute error of band gaps by 50% when compared with the other two functionals.

A Gibbs operator $e^{-\beta H}$ for a two-dimensional (2D) lattice system with a Hamiltonian $H$ can be represented by a 3D tensor network, with the third dimension being the imaginary time (inverse temperature) $\beta$. Coarse graining the network along $\beta$ results in a 2D projected entangled-pair operator (PEPO) with a finite bond dimension. The coarse graining is performed by a tree tensor network of isometries. They are optimized variationally to maximize the accuracy of the PEPO as a representation of the 2D thermal state $e^{-\beta H}$. The algorithm is applied to the two-dimensional Hubbard model on an infinite square lattice. Benchmark results at finite temperature are obtained that are consistent with the best cluster dynamical mean-field theory and power-series expansion in the regime of parameters where they yield mutually consistent results.

We develop the high-frequency expansion based on the Brillouin-Wigner (B-W) perturbation theory for driven systems with spin-orbit coupling which is applicable to the cases of silicene, germanene, and stanene. We compute the effective Hamiltonian in the zero-photon subspace not only to order $O\left({\omega }^{-1}\right)$ but by keeping all the important terms to order $O\left({\omega }^{-2}\right)$ and obtain the photoassisted correction terms to both the hopping and the spin-orbit terms, as well as longer-ranged hopping terms. We then use the effective static Hamiltonian to compute the phase diagram in the high-frequency limit and compare it with the results of direct numerical computation of the Chern numbers of the Floquet bands and show that at sufficiently large frequencies, the B-W theory high-frequency expansion works well even in the presence of spin-orbit-coupling terms.

An implementation of a small quantum absorption refrigerator in a circuit QED architecture is proposed. The setup consists of three harmonic oscillators coupled to a Josephson junction. The refrigerator is autonomous in the sense that it does not require any external control for cooling, but only thermal contact between the oscillators and heat baths at different temperatures. In addition, the setup features a built-in switch, which allows the cooling to be turned on and off. If timing control is available, this enables the possibility for coherence-enhanced cooling. Finally, we show that significant cooling can be achieved with experimentally realistic parameters and that our setup should be within reach of current technology.

We study nonlinear magneto-optical responses of metals by a semiclassical Boltzmann equation approach. We derive general formulas for linear and second-order nonlinear optical effects in the presence of magnetic fields that include both the Berry curvature and the orbital magnetic moment. Applied to Weyl fermions, the semiclassical approach (i) captures the directional anisotropy of linear conductivity under a magnetic field as a consequence of an anisotropic $B^2$ contribution, which may explain the low-field regime of recent experiments; and (ii) predicts strong second harmonic generation proportional to $B$ that is enhanced as the Fermi energy approaches the Weyl point, leading to large nonlinear Kerr rotation. Moreover, we show that the semiclassical formula for the circular photogalvanic effect arising from the Berry curvature dipole is reproduced by a full quantum calculation using a Floquet approach.

Nonthermal dynamical critical behavior can arise in isolated quantum systems brought out of equilibrium by a change in time of their parameters. While this phenomenon has been studied in a variety of systems in the case of a sudden quench, we consider here its sensitivity to a change of protocol by considering the experimentally relevant case of a linear ramp in time. Focusing on the $O\left(N\right)$ model in the large-$N$ limit, we will show that a dynamical phase transition is always present for all durations of the ramp, and we discuss the crossover between the sudden quench transition and one dominated by the equilibrium quantum critical point. We show that the critical behavior of the statistics of the excitations, signaling the nonthermal nature of the transition, is also robust. An intriguing crossover in the equal-time correlation function, related to an anomalous coarsening, is also discussed.

It has been recently proposed that the reduced density matrix may be used to derive the order parameter of a condensed matter system. Here we propose order parameters for the phases of a topological insulator, specifically a spinless Su-Schrieffer-Heeger (SSH) model, and consider the effect of short-range interactions. All the derived order parameters and their possible corresponding quantum phases are verified by the entanglement entropy and electronic configuration analysis results. The order parameter appropriate to the topological regions is further proved by calculating the Berry phase under twisted boundary conditions. It is found that the topological nontrivial phase is robust to the introduction of repulsive intersite interactions and can appear in the topological trivial parameter region when appropriate interactions are added.

Recent reports suggested that InN is a highly unusual III-V semiconductor, whose behavior fundamentally differs from that of others. We therefore analyzed its intrinsic electronic properties on the highest available quality InN layers, demonstrating the absence of electron accumulation at the $\left(10\overline{1}0\right)$ cleavage surface and in the bulk. The bulk electron density is governed solely by dopants. Hence, we conclude that InN acts similarly to the other III-V semiconductors and previously reported intriguing effects are related to low crystallinity, surface decomposition, nonstoichiometry, and/or In adlayers.

Recent angle-resolved photoemission experiments have observed a proximity-induced superconducting gap in the helical surface states of a thin film of the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ grown on a superconducting ${\mathrm{NbSe}}_2$ substrate. The superconducting coherence peaks in the electronic density of states are strongly suppressed when the topological insulator is doped with magnetic Mn impurities, which was interpreted as the complete destruction of helical superconductivity in the topological surface states. Motivated by these experiments, we explore a different possibility: gapless helical superconductivity, where a gapless electronic density of states coexists with a nonzero helical superconducting order parameter. We study a model of superconducting Dirac fermions coupled to random magnetic impurities within the Abrikosov-Gor'kov framework, and find finite regions of gapless helical superconductivity in the phase diagram of the system for both proximity-induced and intrinsic superconductivity. For the latter, we derive universal rates of suppression of the superconducting transition temperature due to magnetic scattering and, for a Fermi level at the Dirac point, a universal rate of increase of the quantum critical attraction strength.

We develop a complete theory for symmetry protected non-Abelian statistics of Majorana Kramers' pairs (MKPs) in time-reversal (TR) invariant topological superconductors, with fundamental results being uncovered. By introducing an effective Hamiltonian approach to describe the braiding of MKPs, we show that the non-Abelian braiding is protected when the effective Hamiltonian exhibits a new TR-like antiunitary symmetry, which is satisfied if the system is free of dynamical noise. Importantly, even the dynamical noise cannot cause error in braiding, unless the noise correlation function breaks a dynamical TR symmetry. This is a profound result and generalizes the TR symmetry protection of MKPs to the dynamical regime. Moreover, the resulted error by noise is shown to be a higher-order effect, compared with the decoherence of Majorana qubits without TR symmetry protection. This study completes the theory of symmetry-protected non-Abelian statistics of MKPs, and shows that the non-Abelian braiding of MKPs is well observable and may have versatile applications to future quantum computation technologies.

Superconducting flux qubits are a promising candidate for realizing quantum information processing and quantum simulations. Such devices behave like artificial atoms, with the advantage that one can easily tune the “atoms” internal properties. Here, by harnessing this flexibility, we propose a technique to minimize the inhomogeneous broadening of a large ensemble of flux qubits by tuning only the external flux. In addition, as an example of many-body physics in such an ensemble, we show how to observe superradiance, and its quadratic scaling with ensemble size, using a tailored microwave control pulse that takes advantage of the inhomogeneous broadening itself to excite only a subensemble of the qubits. Our scheme opens up an approach to using superconducting circuits to explore the properties of quantum many-body systems.

We investigate the evolution of magnetic properties as a function of hydrogen doping in the iron-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ using dynamical mean-field theory combined with density-functional theory. We find that two independent consequences of doping, namely the increase of the electron occupation and the structural modification, have the opposite effects on the strength of electron correlation and magnetism, resulting in the minimum of the calculated magnetic moment around the intermediate doping level as a function of $x$. Our result provides a natural explanation for the recent, puzzling experimental discovery of two separated antiferromagnetic phases at low and high doping limits. Furthermore, the increase of the orbital occupation and correlation strength with doping results in reduced orbital polarization of $d_{xz/yz}$ orbitals and an enhanced role of the $d_{xy}$ orbital in the magnetism at high doping levels, and their possible implications on the superconductivity are discussed in line with the essential role of the magnetism.

We construct exactly solvable models for a wide class of symmetry-enriched topological (SET) phases. Our construction applies to two-dimensional (2D) bosonic SET phases with finite unitary on-site symmetry group $G$ and we conjecture that our models realize every phase in this class that can be described by a commuting projector Hamiltonian. Our models are designed so that they have a special property: If we couple them to a dynamical lattice gauge field with gauge group $G$, the resulting gauge theories are equivalent to string-net models. This property is what allows us to analyze our models in generality. As an example, we present a model for a phase with the same anyon excitations as the toric code and with a ${\mathbb{Z}}_2$ symmetry which exchanges the $e$ and $m$ type anyons. We further illustrate our construction with a number of additional examples.

The Weyl semimetal exhibits various interesting physical phenomena because of the Weyl points, i.e., linear band crossings. We show by Floquet theory that a linearly polarized light applied to a band insulator can induce controllable Weyl points. In a tight-binding model, we classify different types of photoinduced Weyl points that lead to a rich phase diagram characterized by the Chern number defined on each momentum slice of the bulk states. Taking into account the nonequilibrium electron distribution, we calculate and explain the nonmonotonous anomalous Hall conductivity in terms of the light frequency controlled shift of Weyl points' position, which also allows us to examine the conductivity's dependence on the driving electric field.

An electronic structure of a Au atomic chain on Ni(110), which was investigated decades ago, has been reinvestigated by high-resolution spin- and angle-resolved photoemission spectroscopy. Clear evidence of a Rashba spin split, i.e., spin-polarization reversal between positive and negative wave vectors with respect to the symmetry point, has been observed in the electronic bands possessing quasi-one-dimensional character. The observed spin-split state caused by spin-orbit interaction (SOI) was overlooked in the previous spin-resolved photoemission study probably due to the dominant effect of exchange splitting of the Ni substrate than the Rashba effect and poorer resolution of the previous measurement. The system containing both the Rashba-type SOI and the exchange interaction with one-dimensional character will provide a platform to investigate the effect of exchange interaction on the spin-split states caused by SOI with low dimensionality.

Using kinetic Monte Carlo simulations, we show that molecular morphologies found in nonequilibrium growth can be strongly different from those at equilibrium. We study the prototypical hybrid inorganic-organic system 6P on $\mathrm{ZnO}\left(10\overline{1}0\right)$ during thin film adsorption, and find a wealth of phenomena, including reentrant growth, a critical adsorption rate, and observables that are nonmonotonous with the adsorption rate. We identify the transition from lying to standing molecules with a critical cluster size and discuss the competition of time scales during growth in terms of a rate-equation approach. Our results form a basis for understanding and predicting collective orientational ordering during growth in hybrid material systems.

While two-dimensional symmetry-enriched topological phases $\left(\text{SET}\mathrm{s}\right)$ have been studied intensively and systematically, three-dimensional ones are still open issues. We propose an algorithmic approach of imposing global symmetry $G_s$ on gauge theories (denoted by $\text{GT})$ with gauge group $G_g$. The resulting symmetric gauge theories are dubbed “symmetry-enriched gauge theories” $\left(\text{SEG}\right)$, which may be served as low-energy effective theories of three-dimensional symmetric topological quantum spin liquids. We focus on $\text{SEG}\mathrm{s}$ with gauge group $G_g={\mathbb{Z}}_{N_1}\times {\mathbb{Z}}_{N_2}\times \cdots$ and onsite unitary symmetry group $G_s={\mathbb{Z}}_{K_1}\times {\mathbb{Z}}_{K_2}\times \cdots$ or $G_s=\mathrm{U}\left(1\right)\times {\mathbb{Z}}_{K_1}\times \cdots$. Each $\text{SEG}(G_g,G_s)$ is described in the path-integral formalism associated with certain symmetry assignment. From the path-integral expression, we propose how to physically diagnose the ground-state properties (i.e., $\text{SET}$ orders) of $\text{SEG}\mathrm{s}$ in experiments of charge-loop braidings (patterns of symmetry fractionalization) and the mixed multiloop braidings among deconfined loop excitations and confined symmetry fluxes. From these symmetry-enriched properties, one can obtain the map from $\mathsf{\text{SEG}}\mathrm{s}$ to $\mathsf{\text{SET}}\mathrm{s}$. By giving full dynamics to background gauge fields, $\mathsf{\text{SEG}}\mathrm{s}$ may be eventually promoted to a set of new gauge theories (denoted by ${\mathsf{\text{GT}}}^{*})$. Based on their gauge groups, ${\mathsf{\text{GT}}}^{*}\mathrm{s}$ may be further regrouped into different classes, each of which is labeled by a gauge group $G_g^{*}$. Finally, a web of gauge theories involving $\mathsf{\text{GT}},\mathsf{\text{SEG}},\mathsf{\text{SET,}}$ and ${\mathsf{\text{GT}}}^{*}$ is achieved. We demonstrate the above symmetry-enrichment physics and the web of gauge theories through many concrete examples.