Topological quantum phases cannot be characterized by local order parameters in the bulk. In this work, however, we show that nonanalytic signatures of a topological quantum critical point do remain in local observables in the bulk, and manifest themselves as nonanalyticities in their expectation values taken over a family of nonequilibrium states generated using a quantum quench protocol. The signature can be used for precisely locating the critical points in parameter space. A large class of initial states can be chosen for the quench, including finite temperature states. We demonstrate these results in tractable models of noninteracting fermions exhibiting topological phase transitions in one and two spatial dimensions. We also show that the nonanalyticities can be absent if the gap closing is nontopological, i.e., when it corresponds to no phase transition.

Symmetry plays a fundamental role in our understanding of both conventional symmetry breaking phases and the more exotic quantum and topological phases of matter. We explore the experimental signatures of symmetry enriched U(1) quantum spin liquids (QSLs) on the pyrochlore lattice. We point out that the Ce local moment of the newly discovered pyrochlore QSL candidate ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, is a dipole-octupole doublet. The generic model for these unusual doublets supports two distinct symmetry enriched U(1) QSL ground states in the corresponding quantum spin ice regimes. These two U(1) QSLs are dubbed dipolar U(1) QSL and octupolar U(1) QSL. While the dipolar U(1) QSL has been discussed in many contexts, the octupolar U(1) QSL is rather unique. Based on the symmetry properties of the dipole-octupole doublets, we predict the peculiar physical properties of the octupolar U(1) QSL, elucidating the unique spectroscopic properties in the external magnetic fields. We further predict the Anderson-Higgs transition from the octupolar U(1) QSL driven by the external magnetic fields. We identify the experimental relevance with the candidate material ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and other dipole-octupole doublet systems.

In this work, a quantum dot that is defined asymmetrically by electrostatic means induced on a GaAs/AlGaAs heterostructure is investigated to unravel the effect of geometric constraints on the formation of spin droplets under quantized Hall conditions. The incompressibility of the excited $\nu =\frac{5}{2}$ state is explored by solving the Schrödinger equation within spin density functional theory, where the confinement potential is obtained self-consistently utilizing the Thomas-Fermi approximation. Our numerical investigations show that the spatial distribution of the $\nu =2$ incompressible strips and electron occupation in the second lowest Landau level considerably differ from the results of the laterally symmetric quantum dots. Our findings yield two important consequences: first, the incompressibility of the intriguing $\nu =\frac{5}{2}$ state is strongly affected by the asymmetry, and second, since the Aharonov-Bohm interference patterns depend on the velocity of the particles, asymmetry yields an additional parameter to adjust the oscillation period, which imposes a boundary condition dependency in observing quasiparticle phases.

We perform combined density functional and dynamical mean-field calculations to study the pyrochlore iridates ${\mathrm{Lu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, and ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. Both single-site and cluster dynamical mean-field calculations are performed and spin-orbit coupling is included. Paramagnetic metallic phases, antiferromagnetic metallic phases with tilted Weyl cones, and antiferromagnetic insulating phases are found. The magnetic phases display all-in/all-out magnetic ordering, consistent with previous studies. Unusually for electronically three-dimensional materials, the single-site dynamical mean-field approximation fails to reproduce qualitative material trends, predicting in particular that the paramagnetic phase properties of ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ are almost identical, although in experiments the Y compound has a much higher resistance than the Eu compound. This qualitative failure is attributed to the importance of intersite magnetic correlations in the physics of these materials.

Recent studies on crystal facet manipulation of anatase ${\text{TiO}}_2$ in photocatalysis have revealed that reduction and oxidation reactions preferably occur on (100)/(101) and (001) facets, respectively; however, a fundamental understanding of their origin is lacking. Here, as a result of first-principles calculations, we suggest that a dissimilar trend in the anisotropy of electron vs hole bulk transport in anatase ${\text{TiO}}_2$ can be a dominant underlying mechanism for the difference in photochemical activity. Photoexcited electrons and holes are driven to different facets, i.e., electrons on (100)/(101) and holes on (001), leading to the observed preference for either reduction or oxidation. This trend of electrons vs holes found in pure ${\text{TiO}}_2$ applies even for cases where a variety of dopants or defects is introduced.

The recent discovery of the Lorentz symmetry-violating “type-II” Weyl semimetal phase has renewed interest in the study of Weyl physics in condensed-matter systems. However, tuning the exceptional properties of this novel state has remained a challenge. Optical lattices, created using standing laser beams, provide a convenient platform to tune tunneling parameters continuously in time. In this paper, we propose a generalized two level system exhibiting type-II Weyl points that can be realized using ultracold atoms in an optical lattice. The system is engineered using a three-dimensional lattice with complex $\pi$ phase tunneling amplitudes. Various unique properties of the type-II Weyl semimetal such as open Fermi surface, anomalous chirality, and topological Fermi arcs can be probed using the proposed optical lattice scheme.

The von Neumann entanglement entropy is used to estimate the critical point $h_c/J\simeq 0.143\left(3\right)$ of the mixed ferro-antiferromagnetic three-state quantum Potts model $H={\sum }_i[J(X_i{X}_{i+1}^2+X_i^2{X}_{i+1})-h{R}_i]$, where $X_i$ and $R_i$ are standard three-state Potts spin operators and $J>0$ is the antiferromagnetic coupling parameter. This critical point value gives improved estimates for two Kosterlitz-Thouless transition points in the antiferromagnetic ($\beta <0$) region of the $\mathrm{\Delta }-\beta$ phase diagram of the three-state quantum chiral clock model, where $\mathrm{\Delta }$ and $\beta$ are, respectively, the chirality and coupling parameters in the clock model. These are the transition points ${\beta }_c\simeq -0.143\left(3\right)$ at $\mathrm{\Delta }=\frac{1}{2}$ between incommensurate and commensurate phases and ${\beta }_c\simeq -7.0\left(1\right)$ at $\mathrm{\Delta }=0$ between disordered and incommensurate phases. The von Neumann entropy is also used to calculate the central charge $c$ of the underlying conformal field theory in the massless phase $h\le h_c$. The estimate $c\simeq 1$ in this phase is consistent with the known exact value at the particular point $h/J=-1$ corresponding to the purely antiferromagnetic three-state quantum Potts model. The algebraic decay of the Potts spin-spin correlation in the massless phase is used to estimate the continuously varying critical exponent $\eta$.

We present numerical evidence that the spin-1/2 Heisenberg model on the two-dimensional checkerboard lattice exhibits several magnetization plateaus for $m=0, 1/4, 1/2$, and $3/4$, where $m$ is the magnetization normalized by its saturation value. These incompressible states correspond to somewhat similar valence-bond crystal phases that break lattice symmetries, though they are different from the already established plaquette phase for $m=0$. Our results are based on exact diagonalization as well as density-matrix renormalization-group large-scale simulations and interpreted in terms of simple parameter-free trial wave functions.

The main object of investigation in magnonics, spin waves (SWs) are promising information carriers. Presently, the most commonly studied are plane-wave-like SWs and SWs propagating in confined structures, such as waveguides. Here we consider a Gaussian SW beam obliquely incident on an ultranarrow interface between two identical ferromagnetic materials. We use an analytical model and micromagnetic simulations for an in-depth analysis of the influence of the interface properties, in particular the magnetic anisotropy, on the transmission of the SW beam. We derive analytical formulas for the reflectance, transmittance, phase shift, and Goos-Hänchen (GH) shift for beams reflected and refracted by an interface between two semi-infinite ferromagnetic media. The GH shifts in SW beam reflection and transmission are confirmed by micromagnetic simulations in the thin-film geometry. We demonstrate the dependence of the characteristic properties on the magnetic anisotropy at the interface, the angle of incidence, and the frequency of the SWs. We also propose a method for the excitation of high-quality SW beams in micromagnetic simulations.

We report hafnium nitrides under pressure using first-principles evolutionary calculations. Metallic $P{6}_3/mmc$-HfN (calculated Vickers hardness 23.8 GPa) is found to be more energetically favorable than NaCl-type HfN at zero and high pressure. Moreover, NaCl-type HfN actually undergoes a phase transition to $P{6}_3/mmc$-HfN below 670 K at ambient pressure. ${\mathrm{HfN}}_{10}$, which simultaneously has infinite armchairlike polymeric N chains and ${\mathrm{N}}_2$ molecules in its crystal structure, is discovered to be stable at moderate pressure above 23 GPa and can be preserved as a metastable phase at ambient pressure. At ambient conditions (298 K, 0 GPa), the gravimetric energy densities and the volumetric energy densities of ${\mathrm{HfN}}_{10}$ are 2.8 kJ/g and 14.1 kJ/${\mathrm{cm}}^3$, respectively.

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of the three-dimensional topological Kondo insulator ${\mathrm{SmB}}_6$, in particular, near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk ${\mathrm{SmB}}_6$, which produce local magnetic fields of about $\sim 1.8\mathrm{mT}$ fluctuating on a time scale of $\sim 60\mathrm{ns}$. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of $\sim 40–90\mathrm{nm}$, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.

We report on an electron spin resonance (ESR) study of a nearly one-dimensional (1D) spin-$\frac{1}{2}$ chain antiferromagnet, ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$, with extremely weak magnetic ordering. The ESR spectra at $T>T_N$, in the disordered Luttinger-spin-liquid phase, reveal nearly ideal Heisenberg-chain behavior with only a very small, field-independent linewidth, $\sim 1/T$. In the ordered state, below $T_N$, we identify field-dependent antiferromagnetic resonance modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet. Additionally, we observe a major resonant mode with unusual and strongly anisotropic properties, which is not anticipated by the conventional theory of Goldstone spin waves. We propose that this unexpected magnetic excitation can be attributed to a field-independent magnon mode renormalized due to its interaction with the high-energy amplitude (Higgs) mode in the regime of weak spontaneous symmetry breaking.

The observation of nonsaturating classical linear magnetoresistivity has been an enigmatic phenomenon in solid-state physics. We present a study of a two-dimensional ohmic conductor, including local Hall effect and a self-consistent consideration of the environment. An equivalent-circuit scheme delivers a simple and convincing argument why the magnetoresistivity is linear in strong magnetic field, provided that current and biasing electric field are misaligned by a nonlocal mechanism. A finite-element model of a two-dimensional conductor is suited to display the situations that create such deviating currents. Besides edge effects next to electrodes, charge carrier density fluctuations are efficiently generating this effect. However, mobility fluctuations that have frequently been related to linear magnetoresistivity are barely relevant. Despite its rare observation, linear magnetoresitivity is rather the rule than the exception in a regime of low charge carrier densities, misaligned current pathways and strong magnetic field.

Magnetic anisotropies in chiral spin systems can affect dramatically the allowed stable magnetic configurations. Using a Green's function approach, we examine the role of fluctuations in determining boundaries between allowed states in a monoaxial chiral magnet. The effects of uniaxial anisotropy are found to be most pronounced near the point separating the helicoidal, ferromagnetic, and paramagnetic phases. The cone state, which can appear when an external magnetic field is applied, is particularly strongly affected near the boundary with the paramagnetic phase. A metastable region is found, indicating the appearance of a fluctuation-induced phase transition. This metastable region arises from off-diagonal elements in the Green's functions, and can exist even without anisotropy.

We study a possible superconductivity in quasiperiodic systems by portraying the issue within the attractive Hubbard model on a Penrose lattice. Applying a real-space dynamical mean-field theory to the model consisting of 4181 sites, we find a superconducting phase at low temperatures. Reflecting the nonperiodicity of the Penrose lattice, the superconducting state exhibits an inhomogeneity. According to the type of the inhomogeneity, the superconducting phase is categorized into three different regions which cross over each other. Among them, the weak-coupling region exhibits spatially extended Cooper pairs, which are nevertheless distinct from the conventional pairing of two electrons with opposite momenta.

Matrix product operators (MPOs) are at the heart of the second-generation density matrix renormalization group (DMRG) algorithm formulated in matrix product state language. We first summarize the widely known facts on MPO arithmetic and representations of single-site operators. Second, we introduce three compression methods (rescaled SVD, deparallelization, and delinearization) for MPOs and show that it is possible to construct efficient representations of arbitrary operators using MPO arithmetic and compression. As examples, we construct powers of a short-ranged spin-chain Hamiltonian, a complicated Hamiltonian of a two-dimensional system and, as proof of principle, the long-range four-body Hamiltonian from quantum chemistry.

We report on the intrinsic optical generation and detection of coherent acoustic phonons at (001)-oriented bulk Si and GaP without metallic phonon transducer structures. Photoexcitation by a 3.1-eV laser pulse generates a normal strain pulse within the $\sim 100$-nm penetration depth in both semiconductors. The subsequent propagation of the strain pulse into the bulk is detected with a delayed optical probe as a periodic modulation of the optical reflectivity. Our theoretical model explains quantitatively the generation of the acoustic pulse via the deformation potential electron-phonon coupling and detection in terms of the spatially and temporally dependent photoelastic effect for both semiconductors. Comparison with our theoretical model reveals that the experimental strain pulses have finite buildup times of 1.2 and 0.4 ps for GaP and Si, which are comparable with the time required for the photoexcited electrons to transfer to the lowest X valley through intervalley scattering. The deformation potential coupling related to the acoustic pulse generation for GaP is estimated to be twice as strong as that for Si from our experiments, in agreement with a previous theoretical prediction.

We present the studies of structural, magnetic, and magnetotransport properties of ${\mathrm{Cd}}_{1-x-y}{\mathrm{Mn}}_x{\mathrm{Zn}}_y{\mathrm{SnAs}}_2$ crystals with the average Mn contents $x$ changing from 0.013 to 0.170 and Zn contents $y$ varying from 0.002 to 0.051. Homogenous distribution of Mn ions is observed for the samples with $x\le 0.025$. The presence of MnAs clusters in the studied alloy for $x>0.025$ induces room-temperature ferromagnetism with the Curie temperature $T_C$, with values around $325\mathrm{K}$. High crystal quality leads to high carrier mobility values observed for all of our samples, as high as $7100{\mathrm{cm}}^2/\left(\mathrm{V}\mathrm{s}\right)$ for $x=0.025$ and $y=0.028$. The Shubnikov–de Haas oscillations are observed at $T\le 50$ K for all of our samples. The oscillations allowed the calculation of the effective mass $m^{*}$, giving values of about $0.11–0.12m_e$. The presence of magnetic impurities has a strong influence on the magnetoresistance of the alloy. For the samples with $x\ge 0.076$, the Shubnikov–de Haas oscillations are observed on the background of a strong linear positive magnetoresistance, present up to room temperature. The maximum values of the linear positive magnetoresistance are close to 200% for the sample with $x=0.170$ and $y=0.002$, at $T=1.5\mathrm{K}$. This positive magnetoresistance is related to the presence of MnAs clusters in the semiconductor lattice.

We present experimental data and associated theory for correlations in a series of experiments involving repeated Landau-Zener sweeps through the crossing point of a singlet state and a spin-aligned triplet state in a GaAs double quantum dot containing two conduction electrons, which are loaded in the singlet state before each sweep, and the final spin is recorded after each sweep. The experiments reported here measure correlations on time scales from 4 $\mu \mathrm{s}$ to 2 ms. When the magnetic field is aligned in a direction such that spin-orbit coupling cannot cause spin flips, the correlation spectrum has prominent peaks centered at zero frequency and at the differences of the Larmor frequencies of the nuclei, on top of a frequency-independent background. When the spin-orbit field is relevant, there are additional peaks, centered at the frequencies of the individual species. A theoretical model which neglects the effects of high-frequency charge noise correctly predicts the positions of the observed peaks, and gives a reasonably accurate prediction of the size of the frequency-independent background, but gives peak areas that are larger than the observed areas by a factor of 2 or more. The observed peak widths are roughly consistent with predictions based on nuclear dephasing times of the order of 60 $\mu \mathrm{s}$. However, there is extra weight at the lowest observed frequencies, which suggests the existence of residual correlations on the scale of 2 ms. We speculate on the source of these discrepancies.